Basement renovation - advice on cement block wall cleaning + insulation, floor-to-wall plastic strip, and strange connection between metal beams and 2×8s

Hello, first time posting here and would appreciate any advice people have to offer if they've dealt with similar remodeling. We'd like to turn our basement into a nicer place to put a workout space and guest suite, but there are a few issues that I want to fix first:

●The concrete block walls appear to have mold, which can be cleaned. But there are holes in some of the blocks, and the corner has some kind of styrofoam in it. I dont know if this foam is the remnant of an old insulation layer that was left from a prior job that can be removed, or what it was for. I also dont know how best to fix the holes, was thinking of scrubbing everything clean and then filling with spray expanding foam, and repainting.

●Where the concrete floor meets the walls, there is a strip of plastic material between the two that sticks out. I keep finding spiderwebs spun between the wall and the inside of the plastic, and Id really like to rip it out and then put a new self-levelling layer down (in particular, to cover the drain hole and level that section of the floor), but Ive never seen this before and would like to know why its there before I do anything.

●Where the 2×8 beams meet the metal support beams holding up the first floor, you can see that it appears somebody stuck regular nails inside the bolt hole along with the bolt (you can see the nail head sticking out between the washer and bolt head). This house passed inspection 3 years ago, but I have never seen this before, and I'm trying to figure out what was done, and if this is safe.

I really appreciate any assistance/advice people can provide, image link below:

https://imgur.com/a/XGRkLn8