The 843BL is a Wall Cap for a 6" duct, but we do not recommend it to be used with model 512M as the duct adapter is too deep. You will have issues making the duct connection work with the 512M. The part number for the replacement wall cap for a 512M is S97011740. You may order this part direct from Broan-NuTone by calling 888-336-3948.
The main issue here is a dryer use a 4 inch duct. This wall cap is for 6 inch ducting. The secondary issue is that this wall cap has a non removable wire screen to prevent birds and rodents from nesting in the duct. Lint will build up on the screen and become a fire hazard and prevent the damper door from closing causing air drafts into the home. Technically you could use it the way you describe if you ignored all the problems it would create.
Yes, smaller piping has too much resistance and gets too noisy
Broan-NuTone products can be shipped via and expedited service, but you will have to request that when you enter an order on the Home Depots site.
The 843BL is designed for an exhaust fan, but not for dryers.
Mine is same color, matches house color. Can be painted other colors.
Doubtful. I have installed generic wall caps with the same problem. The spring mechanism on the backdraft damper (the flap that keeps air out) of this unit is much higher quality than generic versions. I installed in a windy location and have had no issues with it. Would highly recommend.
You can reduce your ducts down to 6" at this cap but it will just reduce the air flow. There are other 7" vents available on the market that would work for you.
There is a flap that opens when the unit blows the forced air outwards. It also has a grate/mesh behind the flap for critter control.