I'm sure that the old man stepping out of the limo was the wedding singer.

If you recall, there's a pause after everyone elses leaves their cars before the old man is helped out, probably because FFC wants us to see only him, as a reminder that we saw him earlier in the wedding scene.

I don't, BTW, believe that he was at the meeting of the Dons.

As far as him being Altobello goes, I agree with what someone else recently said:

The Altobello character was dreamed up strictly for GF III