Check the rear mount again, you should have 5 bolts total for this mount.
1 X M12 w/o washer - This goes through the bracket, the block and into the trans
3 X M10 - These go through the mount bracket into the trans, one is very short and hidden in the mount. I think the bolt you are missing is one of the two long bolts.
1 X M10 - This goes through the mount bracket and through the motor mount on the crossmember.
There is also one additional M12 bolt with a washer that goes through the block into the trans, this does not touch the mount.