Excellent work so far! (I've been busy and haven't checked in for a bit).
Silicone: yes, grey high-strength silicone gasket maker/sealant is the stuff.
Backplate wobble: normal.
Before you apply silicone to the second axle, install it dry and check the end play. It shouldn't have changed, but if it has you might need to add/remove shims to bring it back into spec.
When I installed the retainers I went full redneck: put the axle in the freezer overnight, heated the retainer with a heat gun, and then walloped the retainer into place with a BFH and length of scaffold pipe.