Questions about: rust-removal

You actually have two problems, starting with how to get the post out of the sleeve now. The first thing to try is some force, and the easiest form of force here would be a hammer. Circle the post, whacking it right above where it enters the sleeve (
You are of course, right to be concerned with this column. It is supporting the floor above it, and would cause significant damage to your home if it failed. It is unclear how long the current one will last if left alone
None of the above? It's structural steel, you can just leave it rusty (if it happens to be corten, the rust actually keeps it from rusting further. .
If you wish to actually de-rust the tub, you have a few options, most of which are slightly destructive, to remove the rust. Wire wheels or other powered grinding/sanding devices will be quickest, but remove the most material. Hand sanding/abrading w