Absolutely no problem to change his card to your room. Do this BEFORE he gets his kids bracelet. The kids bracelet is mandatory, has nothing to do with these bracelets you can buy.
Yes correct, done this many times.
I am not sure about the Mera, but you are aware that the European Championship starts next weekend and soon after the Copa America? Usually MSC uses the theater for the matches then and no more regular shows. If the shows continue, booking with the app one or two days before should be enough.
Pretty hard to make the decision for myself, impossible to say what is best for you 🙂