Repair 2 small holes in floor

Over the last few weekends I've been trying to remove the seat from my W123, one of the 4 mounting bolts was seized so I kept on applying WD40. The bolts go from the seat frame to a bracket welded to the floor and and the nut is held onto the bracket via a clip, with a captive nut on the underside.

I noticed the WD40 dripping onto the ground and saw that a previous idiot (not the guy I bought from I'd say) of an owner had used a much longer than standard bolt which was protruding through the floor. I'm unsure if it punched though as it was tightened, or if a hole was drilled for it!

So now I have 2 holes in the floor and from what I can see the floor is still solid, maybe my efforts to loosen disturbed underseal over them. I applied rust stop. Given that it's in the seat bracket area, and the seatbelt clips into the seat itself I'm worried about safety.

Obviously this needs a plate to be welded in and new underseal, but for a temp fix I was thinking to drill out the holes to clean the area (M8 bolt so use 10 or 12 mm drill), apply epoxy resin, and new underseal.

What do you think of my plan, any other suggestions?

For the underseal will this do?
https://www.vinnybyrne.com/custom-aerosols-paint/rust-conversion-products/stone-chip-protection.html

Pics show top and bottom of floor (with bolt still in place) - I got seat out but forgot to take pic, all looks very solid from top.

Thank you!