Arabic experts - [i]Allahumma[/i] and [i]ahl[/i] vs. [i]al[/i]

Arabic experts, maybe you can solve two major Arabic questions I have been having.

First, what is the difference between “Ya Allah” and “Allahumma”? Many prayers begin with the latter rather than the former, although I have seen both translated the same, namely, “O God!”

Second, what’s the difference between “ahl” and “al”? I read a lot about “Ahl ul-Bayt” or the household of Hazrat Muhammad (sa). But in the salwat, it says, “Allahumma salli 'ala Muhammadin wa ali Muhammad” (bold mine). Shouldn’t it be “wa ahli Muhammad” (bold mine)?

Thanks!

WRS