Wait a second. I want to make sure I've got this right.
So you put your game on hiatus, the other dm started up a campaign with a system you're not interested in for the other players, possibly not inviting you because they know you dislike Pathfinder (? I'm guessing here) and then wants to keep running his game on a schedule.
You come back, ask him
not to run his game on a regular schedule, then don't run at all when he can't (or chooses not to) come? And then you ask him to not run his game again- killing
his game's momentum- so that you can give it another shot.
Then you take it personally when some of the guys prefer his game and are now considering an ultimatum: "If there's a scheduling conflict, play my game or GIT!"
I think you're reacting very poorly here, frankly. No matter how good a friend someone is, if they run I game I don't enjoy I am not going to play it. It doesn't mean I'm not their friend, just that I don't like the game they're running. If there's a better game, perhaps run by another friend, on the same day and time- hell yeah I'm playing that one instead.
One thing you said that particularly struck me:
However, because of the new guy's unwillingness to put off his own game (and, honestly, I can understand that his priorities have shifted so that his own "baby" is more important that mine, but the thing is: he did commit to playing in my campaign first), I still can't get any momentum going.
Behind this statement are two main things that make me frown.
1. "He committed to playing in my campaign first." Like nobody can drop a game that they don't like? What, that means you have a claim on his time?
2. "Because of the new guy's unwillingness to put off his own game... I still can't get any momentum going." Come on! Now we've jumped from "he committed" to playing every other week to he has to play more than that with your game while sacrificing his game? Suddenly he owes you every Friday night?
First of all, this situation isn't him swiping players from you. This is him saying, "Hey, we don't have a game going on while everyone's out of town, so I'll run one myself!" What is he supposed to do, tell the rest of the group, "Sorry, you can't play, you're in punkunui's group, I wouldn't want to create a conflict" and then spend weeks or months gathering up a different group?
Now, the one thing where I feel you do have cause to feel burnt is that they didn't invite you. However, given that we've already established
you don't want to play the game in question anyhow, why would they?
I'm sorry to say that I'm with your wife. I think you're feeling personally slighted by this when it isn't about you personally. It sucks, but giving out ultimatums and throwing people out of your campaign aren't going to help any- if anything, they will drive those folks away. In fact, I'd echo the earlier comment that you're being inflexible. Why not try to run on a different day? Why not offer to trade a session instead of just trying to take them?
Why is everything the other guy's fault? I sure would be interested in hearing from him or the players to get their perspectives on this.
I thought the house rules question/reply was interesting too-
are you using house rules a lot? Have people in the group been complaining about something?
I would find new players and be ready to drop the old ones if they don't show up for a few sessions, but if I were you I'd also adopt a "quorum plays" attitude- if you have a total of six players, you play if a quorum of x are present (I'd go with three, personally). (Of course, you may need to be ready to modify encounters and such to deal with the lower number of pcs.)
Good luck to you, whatever you decide.