Total newbie question, but what line is this? I think it's the jib halyard, just attached to the deck since the jib is on a furler. Not sure what else it might be. It enters the port side of the mast, comes out near the top and then is attached to a chainplate in the deck just aft of the mast.
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I believe you are correct - that's a jib halyard, and it's now a spare, as you have the dedicated roller furler halyard. It won't hurt anything to leave it right where it is, and if you ever get a whisker pole - you can use the halyard for it. Regarding your keel - yes, it can be ground down, and then epoxied to protect it, and then bottom painted. I used coal tar on mine.
The "keel job" is something almost everyone with a C22 has had to go through at one time or another. There are many excellent posts on it in the archives here. I can't imagine how the keel could possibly be "stuck" in the down position permanently - I'd guess it's locked up with barnacles, up close to where it meets the hull. The whole keel hangs on about a 1" bronze pivot pin - so it's certainly able to be raised & lowered once it's cleaned up. With any older C22, it's very wise to replace all the keel hardware (pivot pin, keel hanger bolts-especially, keel cable, etc., but not winch usually, if it's still okay). Be sure to check your shrouds extremely carefully for any broken wires or rust spots, and it's usually a very good idea to replace any rigging that's over 15 years old anyway. All of this is available at Catalina Direct, and it's reasonably priced.
The diver is correct - the boat is really going to sail much slower than it ordinarily would, because of all the growth on the bottom. As you've said, the first reasonable order of business should be to haul the boat out, and clean and inspect the bottom & keel hardware. If the keel were to fall off, the boat would immediately tip over if under sail - so give everything a good look over.
I'd search for some old forum posts in the archives about keel repair and refurbishment, and it will give you a lot of good knowledge about what is involved, what to look for, and how to go about it. You might be okay to take the boat out for a short trip or 2, but I wouldn't really put too much stress on things, especially under sail, until you're able to address some of these issues. The keel hardware & bottom should be 1st priorities, and the shrouds can wait a little bit if they still look okay.
All of this is fixable, and most of it has been done by just about everyone here (literally) - so don't get discouraged, and feel free to ask any questions you have. If you can find someone in the marina with another Catalina, 25 foot or less, they might be willing to give your boat a quick looking over, and point out some things that may not be obvious to someone who hasn't been sailing on one for years. Good luck-