H,
You are right. Its difficult to get a perfect final coat in winter. Chances are, you'll end up doing your cycle as described.
If you are happy with the layer of finish, do exactly as you said, wait 21 days then polish.
Make sure you hang it, don't put it down, even though it feels hard now it will get a ghost pattern on it. Scruitinse the hardness with your fingernail after 21 days, make sure it is hard right through otherwise the polish stage wont work. Sometimes in winter it takes a bit longer....sorry.