I recommend an old-fashioned mechanic's nutcracker. Search the net for "mechanics' nutcracker tool", AKA "Mechanics' Nut Splitter". Many tool vendors make these. These will split a seized nut across the flats, by hand tightening a screw that drives a hardened wedge gizmo. Works in places not accessible to other tools, like saws or grinders. It will destroy the nut, but sometimes releases the bolt without inflicting additional damage on the seized threads.
Then, if possible, replace with new nuts and bolts of solid brass or stainless steel. The additional cost is trivial. However, it is not easy to find solid brass or stainless steel bolts, to fit toilets. It's a mystery to me, why toilet manufacturers continue to install and sell replacement hardware made of steel that is only brass PLATED, which will eventually rust up, leak, and seize. Possibly, to create work for plumbers. More likely, to sell more new toilets, when perfectly good but leaky toilets get broken in efforts to remove corroded bolts with hammer blows, hacksaws, electric vibrating saws, or electric grinding wheels. Doze strategies are just too risky!
First, try an old-fashioned mechanic's nutcracker