Interesting but big surprise I don't fully agree
Monica likes to change her subordinates that have no real unique use to her. She can hire a maid or driver anytime. They will do the job that she needs. Her secretary is different. She did all the administrative work that Monica had no interest in doing herself. Sure she could hire a new secretary but Monica would need to take on those tasks until the new secretary was caught up with her duties. Monica likes and actually treats her secretary well because she is of great use to her.
There is no need to guess why Monica can choose to fire Fred. We the players read every thought that goes through Monica's mind. If she was firing him because of any other reason other than she just looks down on people and enjoys having the power to hire and fire them at will, we would of read about it through her thoughts.
You are right that anything that lead up to those first three days in ep1 and why Monica is the way she is has not been shown or explained to the players. But why she is horrible to everyone and fires her employees at a whim is perfectly explained from her own thoughts. She is narcissistic and believes she is both the most beautiful and intelligent woman on the planet. She enjoys having power over people she considers beneath her. Which is probably almost everyone. She just enjoys being cruel. There is no evidence that she had hidden feelings towards Fred or anyone else in ep1.
If Marcus never called Monica to the police station she would have had replacements for Julia and Fred by the next day and wouldn't have given either another thought.
