I busted mine off a few weeks ago while changing the shocks. I drove it carefully to the dealer.
The options were:
a. drill out and put bolt in.
b. dealer located used trailing arm and put on and then did alignment.
c. New arm dealer put on and then alignment.
I did b since I wanted it done right and I am planning to get lots more miles out of this car. Was like 500 bucks at dealer, with alignment. I thought about doing it, but am glad the dealer did it. It drives great. 500 bucks was a lot, but my wife and kid drive in the car.
z

Just replaced the shocks and mounts on my '96 855 after one of the mounts broke. Removed the shock entirely and drove around for a week without a shock on one side. Made the car squirrly around corners and erratically controllable on potholed roads.

Since one of your bolts is still in-place, you should be fine 'till you get the problem fixed--but fix it soon or you will destroy that new mount.

Noticed that the bolts you are talking about seem too long for their intended purpose and, as a result, accumulate crap in their exposed threads. When you remove them for the first time, you can, if you are not careful, damage the threads in the welded nut.

I'm guessing that is what you did.

I'm also guessing that between damaged threads and probable overtightening, the bolt broke.

You're going to have to drill out/easy-out the remainder of the bolt. I'm not sure how to put this but . . . if you have to ask how to get a bolt out, you probably shouldn't be doing it. A beginer can easily destroy the welded nut, which would be even more involved to repair or replace.