Rerigging in the slip

[Brion Toss]

Hello
Tactical nuclear weapons are effective for this task. If those aren't available, you might try shaped conventional charges, or oxyacetylene at least. Seriously, in tight spaces it can help to have someone on deck loosen the wire so you can rotate the clevis for a usable angle on the cotter, then tighten the wire so the clevis doesn't spin while you are attempting to straighten the cotter. It might also be possible to loosen the wire, then use Knipex "Magic" pliers to hold the head of the clevis pin while you attack the cotter.
I also use dykes, and our rigger's pliers, and Snap-On's cotter pin tool, and whatever else is needed, but I understand that your circumstance, in the tight space under the tang, doesn't leave much room. Worst case for me is if the cotter pin legs break off nearly flush, the head breaks off when you pull on it, and the remnants are jammed in the hole. Then either a teensy drift punch will work, or you can grind/file all flush and take the pin out. Good luck in any event.
Fair leads,
Brion Toss