As far as I can see you should be mailing to a different address:
You should mail your application to:
VFS Services USA Inc.
UKVI Scanning Hub
145 West 45th St Floor 5
New York NY 10036
According to the UK Government website
You raise two questions.
The uk.gov page uses the term "documents" to refer to the whole packet, i.e., the application form, your passport, and supporting documents. I can understand only wanting the passport back, but were I you I would want to avoid a semantic dispute with VFS about whether the word "documents" includes your passport or not. For the ...
If you have a 4 hour layover, you may need to pass through immigration and leave the terminal but that’s easily doable in 4 hours.
Here are the Post Office details at Heathrow:
The airport Post Office is on the first floor of Terminal 3 departures, in the public area before security ( it's near the security control entrance).
Opening hours: Monday ...