Female turtles can lay eggs without seen a male turtle ever (but without a little turtle in it). If they have produced eggs and can not lay them, they become ill and can die (see "eggbound" for more detail).
So you should be happy, your turtle lay the eggs even though she had no place for it.
Maybe they had no appropriate place for it at their old home too, and have learned it sadly that way...
How should an appropriate place for laying eggs be?
It should be filled with soil, maybe mixed with sand or coconut fiber. You can test the right mix, if you dig with a spoon and the walls are stable and no soil slips into the hole.
It should be warm. For example you could use a lamp like this one for the sun place. (In most cases each turtle needs its own sun place, so you need a second anyway.) Additionally one can dig some heat mat into the soil at one end of the place, so the turtle can choose the distance (and so the temperature). (Important to choose a mat that suits to turtle requirements.)
It must have enough room for the turtle to comfortable turn around. (For red eared sliders this could for example be 50 x 50 cm / 19,5 x 19,5 inch.)
Some turtles are very smart: They want a egg laying place, which is above the water level, so their eggs do not getting flooded.
1.How many eggs will she lay?
This turtles will lay between 6 and 17 eggs.
2.How long will it take her to lay them?
This point I do not know. In nature the turtle is vulnerable while she laying eggs, so she want to make it as fast as she can.
3.How can or should I redirect her from laying in the water to laying in soil?
See, if you can optimize the "laying eggs" place. You can not force her to lay her eggs in the ground.
4.How can I tell when she is about to lay another egg?
Under normal circumstances one see it, if the turtle gets agitated and wanders a lot of times to the egg place. She will test to dig the soil and maybe wet it, so it suits more her vision of a good place.
Additionally you can carefully touch her skin and feel the eggs inside her belly. You should be very careful, because "in production" eggs have very soft shell and coud be damaged by humans. The right places to examine, you can see at this picture (The green arrow shows the right place, at the red arrows can you feel bones)
But you should be aware: to touch your turtle and searching for eggs is stressful for her. So you should do it not to often.
For additional information you can look at this questions too:
How long will it take a Red Eared Slider to lay eggs once she is gravid?
Could my RES be egg bound?
