A couple questions about clean up on some projects

Hello, I’m back again. A couple small questions this time, nothing major. But I do have a fun story to tell to explain one of the questions. I’ll put all the questions in bulk at the end though for those who don’t care.

For the story, I ran a new circuit for the lights in my basement. I got that project done fine and it works exactly as expected. But on the last step, when I hooked up the breaker I noticed the 15 amp single pole breaker above it had a white wire, not a black. Ok, not the biggest issue. I just need to figure out where it goes and reverse the wires on the circuit because I thought they had the black on the ground bar instead of the neutral. Instead the black wire went to a 20 amp single pole breaker. But it gets worse.

That side of the house has a lot of electrical stuff that doesn’t work. Several outlets, outside lights etc. it really could’ve gone to anything. But what it went to I wasn’t expecting. We have an electric radiator in our bathroom that hasn’t worked since moving in. I never payed attention because even on the really cold days the bathroom stayed warm since the rest of the house is fine with heating. Turns out the radiator, which requires a 220v circuit on a double pole breaker, was wired with a 12/2 romex NM wire and wired to 2 single pole breakers…. So that’s disconnected now.

My question is, since the slots are pulled out of the panel can I leave these breakers in there not wired to anything? What is the proper way to close those up?

Also, I took out some fluorescent lights and moved them from the window to the center of the room. In doing so I have a good bit of extra cable. It’s not Romex, but it is 600v NM cable. It’s the black rubbery coating. Is it worth saving this in case I need wire for a spare project or is there some kind of risk to using the old cable?