How to loosen a rusted nut/screw
The most expedient and practical solution is to drill out the bolt.  Probably access is better from the top through the tank, but it can be done from either side.

The goal of "drilling out" is to weaken the shaft of the bolt enough so that either the friction on the nut is reduced to the point where it turns, or the shaft breaks and separates into two pieces, both of which are easily removed.

Your concerns of damaging the porcelain indicate the choice of too large a drill bit.  Choose a diameter smaller than the diameter of the bolt's shaft, preferably in the range 60 to 95%.  If kept to the center of the shaft and parallel to it, the drill bit won't touch anything outside the bolt.

In the case of drilling from the top where the head is, drill into it about 6 mm/0.25 inches and then try turning the bolt.  The head should break off and free both pieces.