i dont know the answer to all these questions lol but ill help out the best i can with what i know.
I have a crested gecko, my second reptile and mine is in a komodo plastic faun at the moment until its big enough to go into a glass exo terra.
too much humidity is bad for them, 70-80% is good enough and maybe a little more when shedding. you have to let it dry out in the day and then mist in the evening. being to wet all day can lead to respiratory infections. so a digital thermo/hydro meter will tell you if you have the correct temps and humidity.
not sure about live plants as mine is on artificial at the moment.
repashy in my opinion is the best, people do use other brands, and it depends if your crestie will even like the repashy, mine had no problem moving on to it though.
and yes you can feed locusts instead of crickets, i prefer locusts much more nutritional. and yes you should dust your live food with a calcium and d3 and also gutload your live food as well.
you can give them uv, but remember in the wild they will hide and sleep in the daytime so uv isnt necessary, if you want to i think its a 5.0% one you can use, arcadia lighting is the best.
you can give them a gradient too, i think other people can answer this question, mine just has room temps at the moment even though it is snowing we have the central heating on and the room is still nice about 20 degrees.
hope this helps.