What is this piece in 301 ?? Help.

2-) I couldn't put on the spare wheel fixing stuff in the 3, 4, 5-th photos. The plastic screw sounds short. Or is there another piece under it? There is a black plastic piece that plays back and forth on the bottom of the trunk only, and there is nothing else.

3-) The place that I indicated with the arrow sign in the last baggage photo remains open. So there is something like a plastic cap or something metal with screws?

I need your valuable information, friends. It will be very acceptable. Thanks in advance ..