Baseboard diffusers

Hello, I am replacing the flex ducts on my house with metal pipes and was wondering if you can temporarily mount everything in place and then applying Mastic while the heat is running.

I am replacing a busted elbow at the roof vent for a dryer. Looks like there are two options - 1) I can put a new elbow at the roof so the riser duct remains plumb, 2) or, attach the riser duct directly to the roof vent at an angle (I think the roof pitch is between 30-45 degrees) and adjust the elbow at the bottom of the riser. The second option will eliminate one elbow and make the bottom elbow angle more than 90 (better for flow). I am also thinking of securing that riser with metal pipe strap. Riser is about 6-1/2 ft long. Are there any disadvantages/cons to having long angled riser ducts vs. vertical? (like more lint sticking inside angled vs vertical ducts)