Looking at your photo, I would have put two bearers going across the floorboards to put bath on, avoids any movement from floorboards, & the sealant from cracking.
If you prop up bath you might stress it & get a even bigger split, most likely in the bath, when it's full.
You might be able to slacken off bath supports, one at a time & slide in a bearer, l used some 18mm ply offcuts I had & laminated 3 pieces together, 5x2 would be wide enough, might get away with 4x2, but I always prefer it wider to spread load.
Should be able to remove the sealant carefully in affected area & reseal.