Can't help with your main question, the 1 year start, but I can with the others.
You don't hand in your US licence (as you're not exchanging it,) so there's no issue with getting it back. As far as the UK is concerned you are applying for a licence from scratch so you follow the same process as a new driver; provisional licence (learners permit) and all the tests.
If you have a Canadian licence then you can just swap it for a UK one, citizenship doesn't matter. I believe they will take the Canuck licence in this case as it's a swap but if you move back to Canada you could then hand in your UK licence and swap back to a Canadian one.