Can plus ones answer my custom RSVP questions?

Yes. Plus ones will be asked the same questions as the guest they are a plus one of.

If you want your plus ones to be asked a different set of questions, or otherwise have a different level of access, you can do that too. You just need to add them as a named guest. This necessitates finding out the name of the plus one or just adding them in as "Janes guest" in the name field.