Kroil. The stuff works where no other penetrant will.
Time for new Calipers - just remove old and install new. You are paying now for your lack of proper maintenance in the past.
I second the suggestion on Kroil. It is by far the best penetrating oil that I have used. Give it a little time to work then heat the caliper and then spray the bolt with something with something that evaporates quickly that will cool it. It is literally probably a crime to suggest it these days, but I made a fitting to spray from little automotive Freon cans. Use an impact wrench before the temperature between the bolt and the caliper get a chance to even out.