Cover the spot with baby powder or cornstarch and leave it sitting for a few days. It will draw out the oils

No. Some 4x4 tile mfrs ship them in sheets and the webbing keeps the proper spacing during installation (just as it does with smaller or patterned tiles). Be sure to check for a pattern by spreading out a bunch in a square (I suggest at least 3’x3’) so you get a good look at how they match or relate to each other. If not, you run the risk of noticing too late, or worse having a visitor (like your father in law) point it out to you after the job is done.

You need to add a no other 2x4 to support the end against the wall. You can do this from the other side of the wall if possible. If not, make a 2x4 3”-4” longer than the opening. Put it under and first attach the existing floor from the top. Then put the subfloor back on top and screw it down.

In the US most states and the EPA recognize that concrete, bricks and broken blocks from foundations are inert enough to be used as clean fill or crushed and used as aggregate.