LOL! Never apologize for asking questions! Look at it this way - posting your questions up here will likely help someone in the future with a similar problem - only they'll find an answer (hopefully!)
I am so happy you've seen him eat! HUZZAH! Very important to be sure of this, obviously!
Frogs seem to be really easy to train - mine eats right out of my fingertips! I've read many instances where people feed in a cup or a dish of some sort, so I'm fairly sure this will work out for both of you. I don't think it matters if you keep the cup in the tank or not, as long as you're consistent in always placing his food in the same spot - and it will make things easier for you to clean. I'd say it's up to you if you want to have a cup in your tank, lol! Personally, I'd want to get something smaller to keep in-tank (many people recommend the water catchers from the small terracotta flower pots) and clean up after meals with a turkey baster - I worry about contaminating my tank with putting things in and out every single day - but I don't think it will matter to the frog, as long as his water stays clean and he gets something to eat! Whichever way you decide, once he associates food with that cup, that's where he'll go to get some! Keep in mind that you must be very careful never to get soap into his tank or he will die. Wash the cup with vinegar and rinse it very well between uses.
As for rubber-bands, I'm not entirely sure! I've seen people recommend them for use in attaching plants to driftwood and such, but I used fishing line in my tanks. Eventually the band will rot away, I suspect, and need replacing. I have no idea what kind of weird contaminates might go into making those, so. . . *I* wouldn't do it - but that doesn't mean it isn't okay! Gizmo could better answer this one, she has more experience. . .