Good Questions.
1.) You are confusing MICHELE "MIKE" SABELLA of the later donnie brasco era and his (I believe Uncle or at least older cousin) DOMENICO "MIMI" SABELLA. Mike was sometimes referred to as "Mimi" as well. Yes, they were related to the original Boss SALVATORE SABELLA.
Being on the losing side of the Castellamarese War, he was not seen as part of the new guard by LUCIANO. He was probably asked to retire and because of the impending violence chose wisely. And Although Bonanno would be losing a Castellamarese ally "in theory", he probably felt safer being able to remove a future problem.

2.) I don't know who actually sponsored ANGELO BRUNO for membership.
I know his ties to the like of Carlo Gambino go back to prohibition, so its safe to he was a ringer to be made.... He did take the "criminal alias" "BRUNO" after an early family power JOE "Bruno" DOVI. ANGELO was also taken under the wing of Nj Gambling Chief MARCO REGINELLI. HARRY RICCOBENE had a pleasant memory of BRUNO coming up. Its also been said that BRUNO was alittle more handy with a gun in his younger days, then given credit for. I'd say he was probably widely endorsed.

3.) I answered in #1.