Any tricks to checking back for Fast Passes if I didn't get what I want? Should I check daily? Any particular times of day?
Just check every so often, modify a few times and see what comes up. If you're after FoP you might have to put a bit more effort in, but most stuff should be easy enough.
Is it possible to view anything on the app or website only? I couldn't figure out anything when I tried on the app last night
I only used the app for everything but I'm sure you can do it on the website too. You just need to go into the FP you have booked and modify. To be honest though, it's a year since we've been and there's been loads of updates to MDE since so I couldn't swear to what it looks like now.