Here's some tips,
1- The Gasket, my 850 initially wanted to pull the gasket clip in the exact location as yours. Since you like to take things apart, just look closer at the clip, you can support it in such a way that will let you insert it further until the "barb" portion of the triangle snaps into the slot. I was able to lock mine in. Also, smear a light film of silicone grease around the grill mouth, this lets the gasket slide into position much better when closing so it won't pull on the clips as much.
2- Regarding your paint damage, it's on the barrel part of the grill, not the sheet metal cover on the down draft back box. You would have to replace the grill. You can push for a replacement, or you can look up my posts on insulating the 850 to fix this design flaw. Something you could do in a couple of hours.
OR 3, You can treat this like an offset smoker. I'm seeing that high heat units have hot spots that ruin paint. Since this is really only limited to the rear lower left and right sides on the 850, I would sand off the damaged paint spots, then spray with some grill grate spray oil. Heat the grill and this oil will polymerize and "Blue" the bare metal. Do this regularly and these areas will remain black and rust free. I got this idea from the BBQ Scientist video on the subject.
Lastly, be sure your grill is updated to the latest T2.00.03 config 08.005 as this really stabilized the grill startup temps, no more overshooting temps at ignition.
Good luck, for what it's worth I really love the solid construction of the Timberlines, better than anything I've seen around. But I did feel the need to make my changes to make it better.