You cannot become an officer until at least your 17th birthday, though could go through the process of applying before then. To be honest I think it would be struggle to get through officer INT whilst still at school even if you had managed to acquire the education requirements to commission by that point which is unlikely unless you're in Scotland.
Join the reserves and gain the experience of being a rating and work out what life is going to offer you. If you're off to university / higher education then look at the URNU or the RNR Accelerated Officer Programme (AOP) if you still want to commission. If you're going into work then find out what the lay of the land is there in terms of how reserve friendly they are.
There are 18/19 year old commissioned reservists, they are rare and I'm not convinced they have more fun or get more out of the reserves than the ABs who learn how to use machine guns and drive RIBs and who can sneak in a summer mobilisation.
For someone so young who wants a good career in the RNR I would join as a rating, smash in as much fun stuff as possible for a couple of years and get paid for it, especially if you're doing higher education - then do the AOP during my penultimate summer before the real world and so be able to put a commission on my CV.
1) No idea - I know people who carry on assisting with cadet organisations
2) Yes, the process is relatively painless - there are plenty of students in the RNR. The only question is if your university is near a reserve unit or has an URNU
3) You aren't forced to do specific courses, plenty of people still at school go through INT as ratings. It can take longer because you tend to have to wait for the summer courses at Raleigh
4) Officer training both Reserve and Regular assumes no prior experience, and gives no credit for anything you go in with. I've no idea what CCF leadership training looks like, it might be useful, it might not. Plenty of OCs come from the URNU so have some prior experience. If you act like an idiot about it the national INT staff will very quickly deliver an attitude adjustment.
Your best option is to go along to your local unit and talk things through with the recruiting officer. They should also have a Young Officer Training Officer who it may be possible to speak to.
