Hard going in and out of reverse

^^^^^^ This.
Take it back, because if you mess with it they will tell you it's your problem now.

(Take the whole car so they can see/feel it for themselves) and then pull it and give it back.

I'd tell them to pull it, fix it, and install it, at their cost for messing it up in the first place, or at least pay his labor to remove and re-install for them to fix at no cost...

If they built it right in the first place, it wouldn't have to come out.


Lack of quality on their part shouldn't require more work on yours...

DO IT RIGHT THE FIRST TIME!