Hi Chris,
Ive never done anything as messy as yours and I'm amateur but Im thinking;
Grind out all the damage in keel slot (obvious) vacuum out, wipe clean & acetone as needed.
Warm up the hull bottom so any water that has got in under the ballast dries out (we did our keel in a container under an electric blanket for a week)
Consider tilting the boat slightly on one side and pour a little epoxy resin inside the hull so it can search for any cracks between the hull and the keel box tube. Then mix thickened epoxy (microfibres for strength, but also some collodial silica to stop it dribbling out) and bead it in around the joint.
Get the first coat on quick so it bonds a little onto the epoxy resin youve just put in. -not to much of the thickened epoxy, you can always build up layers later.
Tilt the boat the other way and repeat. This is all to bond it back together.
Level the boat, cut a long strip of cellulose (the covers on old PSSA members handbook are ideal) and use this to profile the hull slot. Stick masking tape across the bottom of the hull damage and fold in to stick on the cellulose.
Pour in (Id suspect very messy) more thickened epoxy to rebuild the hull section. Not easy but a dremel should fair out to the right keel profile.
I don't know whether you need a landing 'block' for the keel to bed down on but if so, 'Delrin' (its a strengthen acetal) is used for rudder bearings so long life, waterproof, absorbs wear. I understand its 3D printable, but thats way above my knowledge.
I wouldn't bother with 'gell coat'. If I wanted a better finish think i'd use paint on 'gell shield' (comes in 2 colours, so you can tell when your re-coating when youve got it covered.) its epoxy so waterproof. Maybe one of those radiator brushes would get you up inside?
I'd also look at the top of the keel and measure down to where it lands on the hull. if the boat ran onto ground and tried to tilt, the top edge of the keel and the matching point on the inside front of the keel box may also have damage.
hope this helps.
ieuan