Excellent research from your side so far.
Good luck...
Correct, pin 3 in connector B on the RNS-E should get the GALA/speed signal
Correct too.
They should be the same except for the speed wire which would be missing in the A4 adapter. Moving the speaker connector over to the other adapter and removing the Bose wire should do the job.
The connector you are talking about is SMB (used on the older RNS-E and Symphony I). It will fit but it doesn't lock and may fall off eventually. But the antenna should work, I have seen two cars so far which had an SMB GPS antenna connected to the RNS-E and it was fine.
Good luck...