Well as one who tends to attempt to delve deep into Howard's psyche (sometimes coming up with shit that is quite comical - and not in a good way), I don't think it is much of a stretch for you to comprehend that maybe, just maybe, he learned a little from his mistakes. If you've ever screwed the pooch on a relationship, and subsequently been given another chance at love; you know that there are probably some things you should change in order to not screw it up again. Repeatedly humiliating your wife on a national radio show is probably one of those things. Now this may not please you and other "fans", but maybe Howard has decided that this time, the fans aren't as important as his personal life. If I were he, I'd have told the bloodsucking leeches to "fuck off" a long time ago.
And no, Sal would not have been able to bust Alison. If Jackie, Fred, Gary and Robin couldn't, a lowlife lackey like Sal certainly couldn't. As far as the callers busting on her goes, they aren't employees, Sal is. Big difference.
PS - No, the joke in question would be a "Neigh" joke. The horseface joke was another joke and it was aimed at Gary.
PPS - Alison was from Boston (Brookline, to be exact).