1. I always use one formation unless there's a team that's playing a specific way and what I'm doing isn't working so I will try to find something to counter that. An example is my current save. I'm playing a 433 and I struggle with teams that play with 2 strikers and there is this team that play with a back 3 an AM and 2 strikers so I decided in some games against them to just move my DM to CB so I match their 3 attacking players with 3 CBs.
2. I don't think it's that important if your players are good and the tactic is good. I've participated in challenges where we change tactics every month and we still do well. It might give you a little boost but it's not going to make bad players play well or a bad tactic perform well.
3. I ignore all that. As long as the player has the attribute to play in any position or role I play them wherever fits my overall tactic.
4. Yes the star ratings are not reliable, they can change based on a lot of things like signing a better player, players form, if you stop playing the player etc.
5. That's not something you have to worry about. Like I said as long as they have the attributes and are familiar with the position you can play them in any role
6. I don't really know this is preference but I might also be weird because I'm willing to throw a lot of money at goalkeepers because I think they can be underrated and you will never understand untill you've had a bad one and then you sign a good one. Even the worst player will score you goals if they get enough chances