No, I don't think you're over thinking it Andy. You care about your cats, and you worry about what they'll feel if you're away for what will after all be quite an extensive period of time.
We're fortunate inasmuchas we have a pet sitter who is a gem - she comes and stays for the whole time we're away, and she's great with both our cat and our dog. We've used her now for many years, and she looks after them exactly as we would. But, if you don't have access to that sort of sitter, I'd be inclined to say I'd ensure they were kept in for the time you're away. That could be tricky for someone who isn't used to the cats and their daily routine.
I kind of understand, because when we acquired Mac, it was the week before we left for a week's holiday. Not ideal.
We went, after much deliberation, but I worried and fretted the whole time we were away, thinking - like you - would he forget us while we were away, would he think he'd been abandoned etc, and that was knowing that our pet sitter would be taking excellent care of him. As it happens, when we got back he was delighted to see us, but had clearly had just as much fun while we were away. Just me, but in your position I think I'd postpone the trip for a little while if you can?
Helen - your boys will throw raucous parties whilst you're away, I'm guessing