I want to accomplish this subframe mount task soon myself. I have the replacement mount kit.
Need more details on what exactly is the problem. Is the big bolt rusted and seized up tight with the subframe mount metal bushing in the center of the rubber block? I think that is the way I interpret the issue.
So, you have the task of getting that loose. Well, I think you need a strong vice to hold the bushing, some penetrating oil, and a propane torch. Start soaking the bolt/bushing interface in penetrating oil - not the whole thing, just some where it needs it down the bolt. Repeat this application for a few days. Then light the torch and heat it up good - clamp it in the vice and try to turn the bolt out of the bushing.
While it may take a few days of application for the penetrating oil to really soak in, you can always try to shortcut this (if time is a factor) by applying the oil over the course of one day and trying the removal later. It all depends on how badly it has seized.
The alternative of course is new bolts and bracket from a dealer. I'm sure they have them or can get them.
Thank you for your post. When I replace my subframe mounts I'm going to coat the big bolt with anti-seize to prevent this from happening, based on what you found.