I saw some say the bartender would never talk... so then why warn him on the way out?

If the police accuse him of accessory to murder, which would make sense since he was right there and clearly allowed his place to be used for the crime, I don't think you could assume he would never talk. Especially since a cop was shot.

I don't really think that's why the line was put in, but it still makes sense.

But more than that I think the reason for the line is to make Pentangelo's death even more painful and to cause him more hurt. That was pretty common in mafia killings. If the ability to make somebody suffer above and beyond was there, it was done.

After that, I agree with Turnbull's timeline.

As for Frankie's brother, I think he was brought in because it was thought that just his presence would remind Frankie of the "right" thing to do. Would Michael have killed him had Frankie still talked? Probably, but the looks given in the courtroom say "honor" was the real driver, not a simple fear of retaliation.