Definitely go with the removable assembly. I made my own.
With the stock crossmember you can't drop the oil pan without removing the engine. It's still not that easy even with the removable assembly, but it is possible.
As to your transmission mounts. Obviously someone has already updated yours. You can have both side and rear mounts concurrently, but you risk breaking a stud on the ZF if things move around too much for whatever reason. I converted mine to side mount only.
I don't think there is any increased chassis stiffness with both mounts given the size and strength of the studs.
I removed the rears so I didn't have to worry about it.