Lastly, there is no confusion about whether or not we are playing the "wrong game." D&D is nothing if not flexible, and I have seen it played in many different styles through the years. The way we play D&D does not require us to choose deep roleplay over powerplay, or vice versa.
I see this more as "the game" being a combat focused game, an interaction game or some blend between the two than as "the game system" used to play the game.
I did what I said I would do; I spoke with them all privately before the next gaming session. I asked them what they thought had happened in the group over the last few months that might cause this rift, and what they think should be done to make it better. Here is what I learned.
I had a really good chat with the first guy, over lunch (we work together). We talked at length about the game, wondering aloud about what had happened and what could be done better. He noticed the shift in playstyle too, and that he thought it had more to do with their characters' levels. Back when they were all low-level characters, combat was a lot more risky and limited, so they had to resort to skill challenges in order to survive. But now, they are all 12th and 13th level, and death is more of an annoyance thanks to Raise Dead and other effects. They also have a lot more combat options, too: new weapons, new abilities, new spells, and so on, that they are just itching to try out. Skill challenges, he said, feel like a waste of time now that there is "cooler stuff" that they could be doing. As for making it better, he got really uncomfortable and changed the subject.
The second guy I spoke to was a live wire...he seemed like he had been waiting for this opportunity. He blamed me for the rift, saying that the difficulty level of their skill challenges is not proportionate to the difficulty level of combat. (A CR 10 trap, he said, is a major challenge that could wipe out the party...but a CR 10 monster is a pushover...they don't want to do skill challenges anymore because they are too hard.) He suggested I curb the difficulty of the skill challenges drastically, or just handwave them. "Every character in the group is built for combat, not skill challenges," he said, "or didn't you notice?" He went on to complain about other stuff, like save-or-die spells or the confirmation roll for critical threats, but in all honesty I could barely listen to him after that "didn't you notice" comment. It was all I could do to remain polite.
I suspect that at least one character is not focused exclusively on combat, and that would be the player that does not want to focus on combat, and nothing but combat. In my games, the more specialized the character, the better he is in his field of expertise, but the narrower that field tends to be. The world does not bend to focus on the specialized challenges he is best suited for, and other characters take the spotlight in situations better suited for their abilities.
The response to his "make the skill challenges easier" request might appropriately be to ask how much easier those should get, with the proviso that the skill challenges drop by the same CR the combat challenge rise by - since the players are all combat-centred, they need more powerful combats to create a challenge. Or maybe the game keeps going with a mix of combat and non-combat challenges. Maybe a group that keeps getting its collective head handed to it by skill challenges might dedicate some character resources to getting better at handling those skill challenges " that could wipe out the party rather than becoming better at handling combat encounters that are already "a pushover".
The third and fourth guys didn't have anything to say at all.
I agree with the comment above that Player 2 seems like the source of the problem. No one else seems to share his complaints, other than perhaps a lesser version from Player 1 (why don't they have any cool new options for dealing with non-combat challenges?).
My girlfriend expressed fatigue more than anything else.
Might be the right answer - it doesn't sound like the other players have any real desire to fix, or even address, the issue, so leave them to their "handwave everything but the pushover combats" game. That also begs the question whether that is the game you want to run, or whether they should find a new GM and you can go play in Ravenloft. Of course, nothing wrong with staying in both games, and kudos to the GF for not trying to move you in lockstep with her.
Tonight's gaming session was rough...it was clear that there were still a lot of hurt feelings at the table, on both sides of the DM screen. But not much else has changed, I'm afraid. Instead of getting off-handed comments about my girlfriend, bias, and favoritism, I get off-handed comments about martyrs, retribution, and vengeance. I'm sure they were just trying to be funny, but I wasn't quite ready to laugh about this yet. So I smiled and joshed and tried to make light of the situation, but my girlfriend was right: it is getting exhausting. I hope this phase passes quickly.
Emphasis added. Sounds like the GF is the mature player. If you're not having fun, why stick around? It sounds like you've made a solid effort to figure out what everyone else wants from the game - what do YOU want from the game? That also needs to be a consideration if the problems are going to be resolved.