How to fix wallplate to very uneven crumbly wall for hangers and joists and a sturdy dance floor?

I would use steel studs resin'd into the stone.

Then you have a lot of leeway for packing and bolting up where you want the wood to be rather than just pulling it tight to an uneven surface.

I'd probably look at M14 at least for that to give plenty of shear strength.

I'd get a structural engineer in to do the calcs. Even more so if the public will be admitted.

You would be better to cut pockets in the stone walls for the joist to enter right in by at least 150mm. Wood preserve and wrap the ends of the joists in plastic to prevent rot

Or use joist hangers. But stone would still have to be removed and cemented back to form a sound base for the hangers to rest on.

No way will you get bolted fixings into walls like those.

I've been intrigued at the responses here, because some years ago I was faced with the same problem.

In my case the stone was very hard, and the mortar very poor, such that when drilling the stones they became loose.

Why a wall plate? Why not use joist hangers pushed into the mortar between course?

Fred wrote: why a wall plate?

because the bolts holding in the wall plate will go into the middle of the stones, and be in irregular spacing across and down, but the hangers will be lined up on a straight line sloping down for drainage.

Hangers don't have to be in line. You can get tall ones, short ones, and for in between the joist can be cut to suit.

That way the joists can still be lined up.