I’ve used an adapter like that on my R1T without issues.

 

Thanks, I really didn't want to rewire the trailers.

 

I've had no issues with either of your questions.
The backup camera works well in aligning the ball to the trailer hitch.
An off-the-self 7-pin to 4-pin adapter works just fine.
The only issue I had was that my utility trailer was too low for the 2 inch drop hitch I owned. I tried a 4 inch drop and even that was too low. I endup with a 6 inch drop hitch, and setting the ride height to "low."
I now have quite the collection of hitches.
Actually, I did have one other issue. The Rivian ball hitch receiver is wider than that on my previous Model X so my hitch lock was too short. I had to buy a longer pin.

 

Backup camera (view/angle) is a few inches to the pass side of the ball, in our experience.

No issue with 7-4pin adapters

To avoid the drop hitch issues others have spoken of, we picked up a weighsafe hitch and have never looked back.

 

I have only towed a small utility trailer and haven't used the camera for aligning but have used a 4pin to 7pin, no problems. I do use the rear facing camera to monitor to the trailer at times, particularly in the beggining to keep an eye on the load.