It's difficult to comment authoritatively on the results you detail Colonel - the only PBEM I've played ended disastrously when my end of the exchange became corrupted. From that limited experience though, I'd already learned that playing against a partner pretty much invalidated almost everything I'd learned playing solo. The "almost" is accounted for by the fact that both modes of play are plagued by the underdeveloped game system. You simply can't believe what the rulebook tells you - and while heroic efforts have been made to patch the holes, the moths still seem to have the lead.
In response to some of your specific observations, it seems clear that the key factor was the Austrian surrender, and from by experience in the single-player version that doesn't surprise. I generally play the CP against the machine and I've found that the quickest route to victory is to roll Russia while occupying a decent number of French provinces. In those circumstances the Russian surrender seems to precipitate the same kind of rush to the peace table as you've noted. It doesn't seem to matter what's happening in the broader context but which particular boxes have been ticked.
As to the submarine issue, I'm figuring that the system isn't so finely tuned to the Lusitania effect as to assume it critical to US entry. Personally I don't find that too concerning. Historically, the politics leading to US entry were so varied and volatile that a lot of dominoes needed to fall before the submarine menace was deemed critical, and they could well have been tipped by the CP successes you describe without the help of torpedoes.
Finally, in regard to the VP situation, I have absolutely nothing to offer. The arcane calculation, reduction, and compensative elements of this part of the game system have always eluded me; so I tend to play as if they weren't there.