I would probably just fill that gap with
epoxy + a strengthener/filler .... or maybe just a 5200 type of product if you are revarnishing. The
wood dries/warps, and holding it flat after that is an effort in futility and likely gains you nothing anyway. If it readily closes up, then Resorcinol or
epoxy glue. But I doubt that.
Stripping in those square holes will be a pain. Just strip what is loose maybe. I'd probably try
paint stripper and a bottle brush.
BTW, white
paint lasts far better than
varnish, so even if you keep the platform varnished it might be worth painting the 'sprit etc.