Basically, once you get to a certain number of players, you could just have everyone claim their role, forcing the Mafia to counter claim a few roles. Then you kill the duplicates until you win. Not announcing roles or even number of Mafia dodges this issue.
So yes, people can make up any kind of role they want. It's usually best to use an actual role as your fake claim, but you can theoretically do whatever you want.
You will not always know if you can trust a role claim, which is why you should watch their overall behavior to see if they seem to have been helping town or not. Sometimes they'll slip up and claim something that someone else knows is wrong as well.
You're also correct that a good Mafia player is going to try not to act noticeably different from game to game. I was stressed out in Star Wars game because while my overall posting frequency stayed fairly consistent, I was mostly just reacting to other people's posts instead of actually trying to hunt down other player's inconsistencies.
As for my vote for today, I think one of the 4 volunteers are likely to be Mafia, and I can't resist odds that good this early in the game.
Vote: Mazre
I feel like Arkos, Tiger, and Mazre have posted similarly, but Arkos and Tiger both came out of the gate with fundamental misunderstandings of how the missions work. I don't think a Mafia player who has had two days to coordinate would have done that. It could just be a ploy, but I doubt it. That leaves Mazre and myself. I obviously won't vote for myself, but I strongly urge everyone to consider the 4 of us carefully, and Mazre and myself especially.