So, I was out doing a few things in a garage, and took another look. I had no trouble getting my Dremel tool to (seemingly) reach the chain, using the large 1-1/4" fiber reinforced cutoff wheel on a mandrel. It should cut the chain no problem, but obviously I'm not gonna test my theory. But, I guess if your tool is bigger in the nose area, it might be too tight. This could be solved with the shaft extension attachment.
If that still doesn't work, then I still think your best bet is to get a 4-1/2" angle grinder with a cutoff wheel and start hacking away at the little "stub" of the hanger bracket under the wheel. You just (just) need to slice off that ~1-1/2" disc at the very bottom, this will also cut the end off the chain.
I lowered my tire and took a look (I have a shop tool for doing this). There are 3 M8 (12mm head) bolts holding the winch to the crossmember. It's very easy to remove (with no tire there), I've had mine off to do the "Rock Warrier" mod. But with the tire in place? No way. Not reachable. As for the entire crossmember, the 4 M8 bolts at the ends are easy. But there is another M8 bolt at the back center that is unreachable. And even if you somehow got a flexy gearwrench on it and got it out, you'll never get the crossmember out with the tire on it. You have to lift it UP slightly to get it out of the end hangers, then slide it back to clear the hangers. The tire will hit the rear crossmember/bumper, and the winch holds the tire against 3 other bumpers/cushions. No way to lift the crossmember.
Fastest way is still with a torch, seriously I've done it and it's a quick "pssssshhh CLANK" (as the hanger bracket hits the floor). Take it to a welding shop, have them cut it off. Just point the torch towards the rear, away from all that scary charcoal canister stuff.
This is why I always hose my spare winch down with lithuim grease while changing the oil and rotating the tires.
An impact driver/gun would be useless for any of this repair. All of the bolts go in from above. No room.
