In general, dogs don't have the mental capacity to connect a reward to any action that happened more than a few seconds ago. If you wanted to teach her a command, you'd have to reward it immediately within a second or two.
However, dogs can learn that certain situations involve treats more often than usual. Some owners give their dogs a treat at the end of a walk. The dog cannot possibly connect the treat to any good behavior during the walk, but it can learn to expect a treat when returning home from a walk.
Your dog can learn that grooming usually yields treats. That little fact may make the process a little more tolerable. Several treats during the session are definitely better than one only at the end of the session.
The study "Reward type and behavioural patterns predict dogs’ success in a delay of gratification paradigm" shows that dogs can understand the concept of delayed gratification, but most dogs choose to be impatient. In the study dogs had free access to a low quality treat, but could see a high quality (or high quantity) treat out of reach. If they waited a certain time, they got access to the better treat, but if they ate the first treat, they didn't get the better treat at all. The maximum time a dog waited for the better treat in this scenario was 140 seconds (just over 2 minutes) mith a mean time of 35 seconds. Your grooming session is a different scenario, but that gives you some numbers to work with.
I wouldn't use the clicker during grooming. Usually a clicker is used to release a dog from the current command, as well as to signal a reward. You say "sit", the dog sits down, you click and the dog is allowed to stand up again and eat her treat. But during grooming you don't want to release her from the session. It might feel frustrating to her to hear the clicker but still have to stand still for grooming.
A 60 minutes brushing session sounds awfully long to me. The problem is not the time, but the activity. Can you remember your parents brushing your hair as a child? For me it was always more painful than brushing my hair myself. If your dogs hair tangles a lot or she picks up burs and other stuff during walks, that means an hour of hair pulling. Her mouthing and wriggling might simply be her way of telling you that what you're doing is uncomfortable or painful. It might also mean that she's simply bored and wants to do something else than standing still to be brushed.
If your brush doesn't have little drops of plastic on the tip of each prong, it may also scratch and irritate her skin. Experimenting with different forms of brushes might be worth it if you find one your dog tolerates better. There are also special "mat cutter" combs that cut through entangled hairs, but please let your professional groomer show you how to use them properly. If used incorrectly, you can hurt your dog more than with a common comb. You can also consider carefully cutting the tangles away instead of untangling them.