I owned an S2a for a short while. I bought it to compare it to my EC's to see which I liked better. I preferred the EC but that is probably because I was used to it. I know some about early Bronica cameras but I am not an expert on them.
The shutter speed dial is a little strange looking. I noticed that on mine but I know of no attachment.
There were no 645 backs for early Bronica's that I am aware of.
I googled the PDF of the instructions to see what you were talking about intermediate shutter speeds. It didn't make sense to me either. I have noticed with some Japanese camera gear the instructions don't always translate well.
Lift the wings and pull up on the connector and it should come off. Remember that these cameras are pretty old.
There is only one helicoid so you have no worries there. Buy any lens you want.
You can check the accuracy of the shutter with a shutter speed tester. Most S2a's bought off Ebay are probably in need of a CLA.
I'm a firm believer in quality tripods rated for at least the weight of your camera. I never had a problem with mirror slap. I don't remember ever shooting at 1/15 or slower but it shouldn't be a problem.
Yes, the matte side of the focussing screen should be facing down towards the mirror and the fresnel (shiny side) pointing up.
There was recently a book published on the early Bronica's you may be interested in.
Good luck to you and I hope I was able to help.