I have had a maple in a pot for 16 years, out in the open and have never watered it in the winter. Under cover? I would suggest you just keep it barely moist as it will be having its winter rest....start watering again in the spring when you see signs of bud growth. I would also suggest you remove the top 3-4 inches of soil and replace with John Innes No3. Hope this helps.

Yes, just water very occaisonally but not when there is about to be overnight frost.

And given where you live, unless the temperature falls to -5C and stays there long enough for the pot to actually freeze, a not very likely possibility, you don't need to give any protection to your Acer.