As long as the subscription has been charged, you can cancel the subscription and the game time from that period’s subscription option will remain on the account.
You aren’t asking complicated questions, these the basics of how subscriptions work. While I tend to answer what I can, a blue response is by no means guaranteed. You can submit a ticket if you want a guaranteed reply from a Game Master, though the answers are available on the site if you use the search option.
It would depend on your timing. A normal subscription only bills when the account has no game time on it. So if you want to do it that way you’d need to wait until all of your game time ran out on the account before attempting to add your subscription, wait for the initial purchase to submit, then remove it.