The main problem is that the head of the bolts with elongated washer arrangements rely on the physical strength of the pressed steel runners to hold them still. This is fine whilst the bolts are not rusty, buts since the bolts protrude underneath the car they are not likely to stay new for any length of time. The heads just end up springing the runner apart and spinning round as you have probably already found out.
Your better off if the old bolts snap off to be honest, as winding the nut down over the length of the bolt is not fun and usually takes two people one to hold the head with some grips and one of you underneath doing the spannering. Then change the bolts for stainless allen key type bolts.
It is not possible to get to the nuts to grind them with a normal angle grinder as they are recessed back into pockets in the floorpan, not unless a small Dremmel is capable of doing the job. On some bolts I managed to get the nut undone a little and using a hacksaw blade alone with a handle I used the flexible blade with very short strokes to cut through the bolts above the nut, again not fun.
All in all it took two of us about 5 hours to get one seat out
I am yet to attempt the other seat
Si21