Keep at it, you will get it done.
The angle grinder with a cut-off can indeed get those hard to remove ones. You may need to cut from the bottom of the head (from inside the bus) down into the foot at a shallow angle and you may need to do this from the other side of the foot as well. Once the angled cut has been made, hit it with a cold chisel and hammer to remove the head of the bolt then a punch to drop the bolt down through the bottom of the floor.
You may even need to cut the foot off and remove the seat to gain access. That is how I will do my NEXT bus as I discovered it on the last few seats.
P.S. I removed all of our seats by myself. I cut them all out. The bottom nuts were too well corroded to easily remove them with a wrench.