I had to look up what “T nuts” are, and on seeing how they work, I think you are out of luck trying to salvage them and to get them apart. Penetrating oil has no prayer of being successful when a nut such as that is anchored in something as soft as foam. Even the best penetrating oils don’t eliminate the need for a lot of torque when breaking stuck threads loose. My guess is that drilling or grinding the exposed bolt head will be necessary, letting you punch the remainder of the bolt (and T nut) out the back side.
Cutting a slot for a screwdriver in the T-nut might be possible, but unless you have a circular cutting wheel you will end up cutting a much longer notch within the foam. And of course, cutting a notch will only work if the end of the bolt is not in close proximity to the back side of the nut. If the bolt goes most of the way through the nut, your screwdriver notch will be incorporated into the bolt too, making it useless.