if you ring me on 07917 035532 I can help.always keep your call,and taking over harsh measures will mean that you can damage it or you.
the nut inside the subframe has either come unwelded or spun.but there is a solution that we do when we are just replacing the wishbones.
simply drill a large hole with a drill on the flat underside of the subframe.make sure its a neat hole.it needs to be a hole large enough to pull the nut through and new one back in.use a magnic to pull and position the new bolt.then you will have to get a plat welded back on.it will only take around 60 minutes to carry this out.
One thing I learnt early on.if something doesnt fit or come off,keep trying the same methods will not get a result.so you need to try differant approaches .
if you remove your subframe it will take a lot longer and have more problems then solving the one problem you have now