Hello there!
1) It is the writer's decision to make Markus seek Fem MC and make Juni seek Male MC & Andro MC. If you want to meet Juni as Fem MC, you can buy the Harmony mod from the clinic to be temporarily androgynous. It will make her available. I'll bring it up to the writer and see if adjustments can be made so they can appear regardless of body type.
2) In regards to Stella, as stated in the patch notes she's fully available only if you chose Fighters as your major at the beginning of the game. Each major will get an exclusive character and thus far the only one we implemented is Stella. The other two majors should receive their exclusive character in one of the upcoming patches.
If you did choose fighters and didn't win the bet during orientation week, there is a catchup/failsafe mechanism. Go to the arcade in the evening of any day and at the bottom you will have a "Join Competition" button. Click on it and you will get the keycard to her room. With the keycard, you can now access her room from the dormitories.
If you chose Cruisers or Battlegroups, you can see her once during orientation but it doesn't go further than that.
3) At the moment there isn't. Later on there will be consequences for cheating but we're not going to implement that until much later, it's very low on the priority list. Right now the only thing you cannot do if you're in a relationship with Julianna is start a relationship with Jaime. The opposite is also true. Everyone else is fair game for the time being, so bang away fellow cadet
Valerie is possible and it's also why she offers it. Just remember that in a future patch you will have to deal with the consequences of that choice if you're in a relationship with either Julianna or Jaime.
I'll bring it up to the writer and see if adjustments can be made so they can at least appear regardless of body type. Feel free to let him know directly on our discord server, it's the best way to provide feedback directly to him.