Certainly keep the fly closed during wet, low-pressure nights, but absolutely open the top third of both front and rear ports of the tent body. Or, try venting one door low and the other high for maximum circulation. Not sure about the stargazer window-you'll have to experiment. Finally, down bags are always risky at such temperatures, especially in New England or boreal forest. Better bag makers use water-resistant shell fabric on their down bags.
Synthetic fills are inherently water-resistant.
If you're reluctant to invest in a synthetic fill bag for wet conditions, consider a simple waterproof-breathable bivy sack, which will extend your camping options in many ways. Zip your techy raincoat around the leg-foot area of your mummy bag. Let me know how it works. You get a gold Star. All rights reserved.