I have removed the subframe bolts twice without problems. Back in 1980 when I restored my very rusty 1975 TA and in 2020 when I removed the subframe on my 1980. Both times I hit them with an impact wrench and they came right off. I think I might have been lucky. In your case if the caged nuts weren't already spinning, I would have suggested soaking them in penetrating oil for a day or two.
While I can't offer any shortcuts on breaking them lose, I strongly recommend you replace the isolators and any of the hardware that is rusty. On my 1975 the bolts were badly rusted and only half the original diameter. By not replacing the bolts I would have been flirting with an eventual disaster.