I think the main focus here is that your cat needs to trust you. You can't just expect it to do so immediately, but I suggest you work at building a rapport with him. I'll focus my answer on the flea treatment, but the principle is the same for all the other things he does not want you to do.
This answer is based on my experience with housebreaking our two incredibly shy cats, who had lived in the wild for their first 6 months. We've had to teach them everything (other than using a litter box), starting from a position where they didn't trust any human. When out in the wild, they lived in a prison, where they survived by avoiding all humans.
Needless to say, they were apprehensive of everything we did (including feeding them, as the prison guards had tried to give them poisoned food). However, over the last 6 months we've managed to gain their trust and turn them into actual pets, so I'm confident that this method works.
Cats don't listen to people like dogs do. They are incredibly independent, and tend to always do what they think is best. Since you can't command a cat to come to you, you should try to incentivize it so that it will want to come to you.
If you want your cat to make a certain choice, then make that choice the best possible choice.
Don't expect a cat to obey you.
- Not one of them will tolerate us giving them any kind of treatment at all
- they would not allow us to even get them into [a harness]!
- as they know what we intend to do and run if I even pick up a towel!
- They are all bad but the elder tom is a nightmare
- He will scratch, bite and struggle so hard to get away that he hurts us regardless of what we wear.
From these quotes, I'm starting to infer that you're expecting your cat to listen to what you want it to do. It might not like the flea treatment, but it should allow it because you're clearly indicating to them that they should allow you.
Cats are not really known for their obedience. It is possible to get them to listen, but this completely hinges on one thing: trust.
Playing the advocate for your cats: what have you done to show your cats that they can trust you?
This isn't a personal attack on you, I hope you don't read it that way. I'm trying to show you what the cats' point of view is. What do they stand to gain from allowing you to do something to them that they don't like? Because if there's no benefit to doing so, why would they want to endure it?
The wrong way
Do not overpower your cat and force him to do something against his will. This may be a quick win for you now, but it will be detrimental in the long run.
If you betray his trust by grabbing him and forcing him, then you're teaching him that it's unsafe to be around you. Over time, he will keep you at a distance, or even hide instead of come to you when you call for him.
Don't try to lure the cat in with treats and then trap him. It may work a few times, but over time he will learn to see it coming, and will refuse to come close to you. He might even end up disliking his treats (or distrusting you when you're giving him treats).
Our cat did this to us. Whenever she saw us handle the cat carrier, even if it was in a different room than me and her were currently in, she would refuse to go near any humans. After a few trips to the vet, she got wise to our tricks to get her into the carrier.
they go wild at even the sight of the flea treatment package!
as they know what we intend to do and run if I even pick up a towel!
This suggests that the cat has bad memories connected to the flea treatment package (the same goes for the towel). Whenever he's seen such a package, he ended up in an uncomfortable situation (wrestling with whoever is trying to apply the treatment).
This is essentially a Pavlovian response. The cat has identified that the uncomfortable moments are always preceded by seeing the flea treatment package, and therefore knows that it's going to be uncomfortable soon (and will avoid it al all costs).
The severity of their wild behavior directly corresponds to the severity of their (bad) experiences.
If your cat requires urgent medical attention, then this does not apply. Medical emergencies are more important than the cat's personal comfort, or your personal relationship with him.
The right way
Doctors give lollipops to children at the end of a doctor's visit. Since this is the last interaction with the doctor, the child will remember the lollipop more than what came before it. The lollipop subconsciously teaches the child to be less afraid of having to go the the doctor.
Essentially, that's what you have to do with your cat. Make the experience (in hindsight) a positive memory, not a negative one.
Every cat, no matter how wild, will always want food when hungry. It's the most basic form of incentivization. Take away his food options except for one food bowl, and sit next to it.
From your description, I gather that he has no problems approaching you (e.g. when you're sitting next to his food), but he puts up a fight once you interact with him in a way that he doesn't like. If your cat is fearful enough that it would rather starve than come near you (we've had a cat like that), then this approach obviously will not work.
When he starts eating, calmly start applying the flea treatment. Act as if this is the most normal thing in the world, don't act like you're pulling off a heist (cats sense the difference, at least ours do)
Given your description of your cat, I expect that he'll stop you (pull away his head). Calmly look at him, and let him continue eating. The moment he continues eating, you continue applying the treatment.
This will repeat several times, and that's okay. Most animals (and children) need repetition in order to learn something. You're teaching him that the food and the flea treatment are a package deal. He gets to make the choice: food and treatment, or no food and no treatment. (Do not starve your cat, see the next chapter)
After a few times, if he's had some food (enough to not starve, but not enough to be full) and still puts up a fight, simply take away the rest of the food and walk away. Ignore him for a minute or two. He will ask for more food at some point (whether it's 2 minutes or a few hours, depends on the cat) When he wants more food, bring the food bowl back and repeat the process.
This may take a while, you might not be able to actually apply the treatment for the first meal or two (where he ends up eating enough in small chunks that he's had his fill).
Eventually though, he should realize that putting up a fight costs him more effort than letting you do your thing.
edit: If you feel like he's not improving, and simply coasting on what little food he can scavenge before you take it away again, then wait longer before giving him more food. Again, do not starve him, but make him wait for food a bit longer. This incentivizes him to behave during feeding time.
Do not starve your cat!
I cannot stress this enough. The point of the exercise is not to punish your cat for not listening to you, you're simply trying to teach your cat to take the bad with the good (food + flea treatment).
If you treat him unfairly (forcing him, starving him, berating him for not wanting the treatment), then he will hold you responsible. This is the opposite of what you want, you're trying to get him to trust you. If your cat trusts you, then he will also trust your judgment.
If your cat is stubborn enough that he would rather starve than let you apply the flea treatment, then you should switch tactics instead of trying to outstubborn the cat.
Give him free access to (a small amount of) cat food that he doesn't really like (but will eat if hungry). However, put his favorite food in the bowl next to you.
This is the same principle as before: the choice is his, does he want the lousy free food, or the good food that comes with the flea treatment?
This is not a quick solution. But it is the best solution in the long run.
Quick fixes often end up making a dent in the cat's trust of you, which will only make future treatments more difficult.
(Unrelated to the problem at hand, but it might help you in the long run.)
You've mentioned that none of the cats allow you to apply the flea treatment. They're probably learning from each other.
For our two cats, if the smarter one gets scared or runs away (e.g. something makes a loud bang), the other will immediately follow suit. The smarter one usually gets over it rather quickly, but the other one can take days before she forgets it. She heavily relies on the judgment of the smarter one (we see that in everything they do, especially when investigating new things)
This can have negative consequences (as you are experiencing now), but you can also turn it to your advantage: if one of the cats allows you to do something that the others don't, immediately reward it. Even petting them for a few minutes (directly after) can signal to the other cats that they are missing out on bonuses.
One more tip: You've mentioned that the cats already turn wild when they see the flea treatment box.
You're going to have to start from a disadvantaged position, as your cat already has bad memories attached to the box/syringe.
If I were you, I would first try to get the cats to forget their earlier memories. Put one of those strips in plain sight, somewhere near their food bowls (or somewhere they often have to pass by), but never use it.
Your cats will be very apprehensive in the beginning (somewhat obviously). But if they see this object every day, and it has never harmed them or been used in any way, and you don't even acknowledge that it exists, then they should stop being afraid of it after a while.