Just to try to hit on these one by one..
The more correct term would be extension ring, they can be used adjust how close or far the port is from the camera and lens.
If the housing isn't setup for Ikelite bulkheads, you won't be able to use those strobes without replacing the bulkheads (approx $150 each??? Been a while, I forget exactly). Otherwise, if you are talking about ball mounts that go on your handles - Ultralight makes solid products, and they are friendly.
I would always recommend vacuum systems, there are a few options. I have used the Vivid leak sentinel and thought it was pretty solid. I have also used the Nauticam, but I'm not sure those can be adapted to Aquatica housings or not.
Generally, I don't think you'll find anyone who shoots the 105 behind a 8" dome. I suppose it's possible, but it's definitely not an optimal combination.
There are several other options to get your housing serviced outside of sending it to Aquatica. I've done it myself, it's not terribly complicated but it takes time. I would estimate $300-500 for a full service (if you haven't noticed, not a cheap hobby).