It occurred to me after I figured out who this NPC noblewoman your allies slaughtered actually is (I'm currently running CoS): How does your DM respond to this behavior?
I suppose it would be reasonable to operate under the theory that your job as DM is to facilitate the game your players want to play. But that NPC is a lynchpin of CoS; I would freak out if my party sociopathed her like that.
Being an online (I assume PbP or the like, rather than live via VoIP) game would, I suppose, insulate the DM from immediate or instinctual reactions and give him time to respond in "good faith" to an epic curveball. Still, does the DM seem to go along happily enough with these shenanigans? If so, I would take that as another piece of evidence that this isn't the game for you.
I can't imagine how I would prepare and run a game that might run so drastically aground on the PCs. I've seen D&D described as a social contract between DM and players where the DM agrees to create a fun setting/story for the players and the players agree to engage with the DM's world (very roughly speaking). This sort of behavior seems to throw that out the window. The players are dishonoring the campaign world by dehumanizing its denizens, and in the process destroying the story.
Now, in this case, that might be residue of the fact that nobody really wants to play CoS, so they're trying to short circuit it, or lash out in frustration - in character. If that's so, the "cooperative" DM response would, I suppose, be to get out of CoS as fast as possible. In any event, I'm very confused about how the DM would deal with a situation like this.
Strahd came and attacked the Rogue for killing her, but since I had immediately started bringing her back he didn’t have much reason to freak out. The second time he killed the Rogue, attacking until he was dead, and continued attacking the party, I also still made it clear my intention was to bring her back that second time as well.
The fact Strahd attacked us for killing her seems to be enough to convince the party that she was lying about being terrified of him and that she was really evil, because why would he attack us if she was a good person who wasn’t working for him (I get why, they don’t it seems)
Also, he has said repeatedly that he is running a modified version of the adventure, to compensate for our power and the fact that this is essentially a sidequest. The more I talk to them though, the less and less I believe they were lashing out against the DM. Frankly, we thought this woman was a maid when we first encountered her, we never learned her name, and only one of us has ever played CoS before and he wasn’t the one inciting the violence. From the perspective of the group, she was literally a faceless NPC with no significance for the plot so killing her would have no ramifications for the game.
You may not 'need' them, but their existence definitely 'helps'. Alignment is used far more often as a justification for atrocious behaviour than in any way that furthers good role-playing - at least in my experience.
The mere existence of 'extreme' alignments (yes, even lawful-good) gives birth to generations of murder-hobos, because it makes susceptible players feel that they 'have to' act in extreme ways, they'd never consider if they had just created a well-rounded character with a believable set of motivations and personality traits: "It's not my fault, my alignment made me do it!"
Alignment is a crutch meant to be a shortcut to actual character development, but in actuality it falls short in every conceivable way.
So, sure, at its root it's a problem with the players, but alignments serve as a welcome tool to legitimate their actions.
None of us use alignment officially in the game. I used Lawful Good as a shorthand for the type of character I was playing. I have no idea what they wrote on their sheets and they’ve never said.
So that isn’t the issue here, according to one of them, they do it for “those dank memes”… which I don’t see how that has a bearing on the decision points I’ve been objecting to, but that’s how it was explained.
A few observations and thoughts:
First, play what you want. If it doesn't get along with the rest of the group, so be it. Fight it out in-character if you have to, but make sure the fighting stays in character. I've seen this many times before - though usually in reverse: one evil character trying to fit in with a bunch of goody-goods - and though the one usually ends up either retiring or dying it doesn't always end that way.
Second, it's interesting that when you suggested switching out your character the other players indicated they liked what you already had. Every good comedy troupe, for example, needs a "straight man" as a foil, and I think that might be the role you're (probably unintentionally) filling here: you're the foil for the rest to riff off of. My worry is that even if you now turn around and bring in a much different character, it'll still become the foil as that's what the other players have come to expect of you, whether rightly or not.
Third, how is the DM forcing you to play the Strahd adventure if you collectively don't want to? Is it a railroad-type game where the players (via their characters) can't throw curveballs at the DM? I ask because if it's not a railroad, what's stopping you from collectively deciding "screw this, we're going after our friend instead"?
Fourth, that lot sounds like the sort of party where I as player would really look forward to the games, just to see what they do next!

Too bad it's online - if it was an in-person game adding a case or two of beer to the mix would be a blast!
Lan-"yes, as a player I'm probably CN-aligned"-efan
2nd: Yeah, I keep thinking that, but then they get frustrated with me being the straight man, telling me I’m too strict and serious and they just wanna have fun. I’m mildly curious how they are going to treat my new guy but he is not going to have the same role as my Cleric, his most severe reaction to the murder of innocents is going to be “that was a waste” because he wants to make a cult and become an archedevil, not that he’s going to tell them that. I’ve never played a character who selfishly wants power before, so it’ll be interesting for me at least. An experiment.
3rd: Well, first of all, my character and the person who left the group were the only two who really considered her a friend. To everyone else she was a rando-NPC. One of the other players, when I expressed concern over her dying if we took too long by going overland instead of buying a boat, pretty much said they could care less if she died or not. But, we also didn’t care about Strahd, they went into the Death House purely because they wanted to kill a monster and the DM placed the house in front of us. To force us to continue he took the soul of the character belonging to the person who left, trapped it in the mist, and said the only way to save her was to kill Strahd. That character is literally the only one all of us care for. We can’t abandon her, in fact, that is one of the only things that bothers me about my character leaving the party. It feels like I’m abandoning her, and the only way it makes sense it to know they won’t abandon her and they kicked Strahd’s teeth in while my character was tied up on the floor, so clearly they don’t need his help. He is going straight to his friend though, and the DM already talked it through with me about how I rescued her and what condition she was in when I found her.
4th: I do like the group, and I do think some of the things they do are funny, I just wish it was more “Bad Company” than “Silence of the Lambs” (I think I got those two movies right)