If I purchased something at TRU and then realized I had gift cards....can I return it and then pay with the gift card?

I see no reason why not. Only issue would be is that your money would take a while to be returned to your bank account or taken off your credit card. So if it's the situation where you want to use the gift cards and spend the money you spent on the original set today to buy something else, it won't exactly work. As long as you don't mind waiting 2 or 3 days (that's the norm in my experience) for the return of the cash, then by all means use the gift cards.

Absolutely.  What you're doing is a refund, and then making a new purchase (with the same item).

yep

I forgot to buy the TRU gift cards at a discount, but couldn't pass up on the item in the store.