It appears that the bolts holding the tub down go into rivnuts, which are undoubtedly turning in their holes, as rivnuts often do when they get rusty. I think your only option is going to be to cut or grind off the heads, and then either destroy and replace the rivnuts, or try to drill out the remains of the bolts. As far as replacing the rivnuts, you may have problems if the holes are enlarged or rusty, or if you don't have access to the correct tool for putting in new rivnuts. It might end up being a two person job, with someone holding nuts beneath while someone else puts the bolts in from above.
