"If they "total " the boat, is their "buy back" price negotiable? "
Everything is negotiable. But be aware that if they total the boat out, whether they transfer it to a salvage title or not (check your state laws) the boat will be a total and you may not be able to get anything more than liability insurance on it afterwards. Check with your insurer and your state about local rules and process.
"Are there ways to mitigate the approximately $300/ foot repaint costs?" Sure, you get bids. Odds are any shop with an environmentally controlled paint booth will charge the same thing. Might be able to fit you in next spring. Guys who work out of a trailer illegally in a field will be cheaper, and some are even good at keeping the gnats out of the paint. Mitigate? Not really, but you can shop around for price and quality. Sometimes you can sail or haul to a shop 200 miles away and simply get a better price.
"What would I need to do to the mast to minimize the effects of the salt water bath?"
Wash in fresh water ASAP. What's done is done and AFAIK
there's no "neutralizing", you just wash it to stop any further salt damage.
" Is it possible to get a slightly used Furler? Are they difficult to install seeing that the mast is down?" There are consignment shops that carry used gear, somewhere on the forums I'm sure there's a list. Hard to install? Nuts and bolts and instructions. Is it hard to build a swing set, tricycle or bbq?
"Would I have any trouble re-surveying the boat after the repairs so I could get the boat re-insured? " Yeah, from the top. Once totalled, some insurers won't touch it. You need to start with your state insurance commission to find out the laws, then ask your insurer, because sometimes they also lie. One of the things they lie about is the value of your boat, again, ask your state insurance commission who or if they are allowed to determine that value, and make sure they have followed the rules. With a car, I've seen an insurer with a top name literally cheat, break the rules, and finally raise their offer nearly 400%.
Think of this as a high-stakes poker game. They may be smiling at you, that doesn't mean they won't take you for everything if they can. If they treat you well and follow the rules--then come back to praise them afterwards. The few good guys deserve that.