back of case screw question

i spilled some liquid and had to pop the case to ensure there was no liquid inside. Luckily there wasn't.

I was very careful in taking it off, however, in putting it back i noticed a couple of things:

1. at the front left i am missing a small black plastic thingie (shaped like a basketball key) that exists on the right side. From what i can tell the front case screws (installed through the battery pack cavity on the bottom) goes through this plastic piece. is there anyway to replace this or do i even need to?

2. Of the 12 small screws for the case 2 are just a little shorter. Do these shorter ones go at the back or are they for the front of the case (accessed through battery pack cavity). (For simplicity, I am ignoring the 4 long, 3 ram cover screws and 2 Torx screws which make up the 21 case screws)

3. 1 of the 2 screws for the back of the case does not sit perfectly. I tried both the shorter and longer case screws mention in #2 above. Neither will sit perfectly. I get it in about 90% in to perfect. I took it all apart again and noticed a clear piece of plastic which i tried to insure was not interfering and then put them all back but the problem still exists.

Thanks