The way to keep the motor mount from loosening up is drill out the threaded holes in the plate with a 3mm bit and run M3x30 button head screws in from the front and a lock nut in the back. Same concept as the Super Shafty and BWD plates but it works well with the stock plate too.
I think that I am going to save the Yeti universal axles for spares for the front end. With a little bit of research, I discovered that original SCX10 diff cups are the exact same that are used for the EXO rear end (AX30366) and I just happen to have a brand new pair of them in my parts stash. I will order Axial EXO Dogbone Set 7x94mm (AX30379) and I will be good to go. Using the dogbone axles in the rear will be good since they obviously won't be used for steering and they are relatively inexpensive; plus they are easy to replace if need be. The 94mm length (vs 92mm) will be welcomed since the dogbones are pretty close to the end of the diff cups in full droop.
There's nothing else I can pre-assemble until the B2B kit arrives.