It may actually be a bit intense for some softies, but the LPS should be fine. Most 10g tanks are 20" wide, and this is 24" wide so it'll stick out on either side of the tank, unless you have some kind of custom tank.
Sure you could. You would just need to think about placement of the softies before you threw them in. I'd put your mushrooms on the bottom perhaps under a rock ledge or something.
Yep. You just need to be aware of it when you're putting them in the tank. My wife upgraded the lights over her 10g to 96w, from 65w. She didn't adjust her photoperiod, and all her leathers bleached out. They're still alive, but they went from bright green to white. They're gradually regaining their color, but it's certainly not the best situation.