Stripped plastic insert for bumper.

Windshield wiper sprayer needs to be cleaned out... I've done it before using a needle

As for rear bumper cover, your description doesn't explain where the gap is when it sags... Part #5 that you want to get might not be the solution if we don't know what it looks like when it sags. I don't see anything that's loose in your picture? Maybe a different camera angle would make it more clear? When I've worked with those plugs, they don't seem to hold much of a load compared to the main bumper cover retainer clip, so it might not be the source of the problem.

In general, Item #6 in the diagram is the most commonly replaced part because when it fails it leaves a gap on the top left edge. You'll see people around that have duct tape and other rather dim-witted repairs for this problem. Here's a decent price on bumper clips: https://www.ebay.com/itm/155032471111

And if it really just is a loose plastic insert just go to the hardware store and find something similar.

 

See my other post again... I had to edit it a bunch of times as you updated your post.

 

I'd replace it with a rivet nut and a flange bolt. It won't sag or come off again.

 

You're saying that black thing the fastener attached to your silver bumper cover right under where the hatch closes is causing a sag problem yeah okay maybe but I kind of doubt it usually it's on the clip that's on the side of the car where the bumper cover bent over factory lip slips in to those push clips of the push clip the black piece mounted to the body of the car but I can't see that in this picture but like above said that other style of fastener would be more permanent and certainly wouldn't sag until the whole wallowed out or whatever happened I'm not sure your rear bumper looks pretty straight even though it's been messed with a little bit it doesn't look too bad but I can't really see the rest of the bumper because that's not what the picture showing