The lack of paragraphs makes your long OP very difficult to read and it's hard to work out exactly what your problem is.
From what I can glean, you felt displaced after the birth of your brother and gravitated towards your df. However, as you've described him as being emotionally detached from the world around him, I suspect his behaviour is not the best yardstick for you to have used in judging your dm whether or not your dm 'shows signs of mania'.
Imo it was presumptious of you to discuss this with your family doctor without first having raised it with your dm or spoken to your df, but at least you were told that it was none of your business only your dm could seek professional help if, indeed, she has mental (or physical) health issues which, it seems to me, she doesn't.
When you dropped your make up in your dm's car while she was driving, she rightly told you to stop trying to pick it up while the vehicle was moving. If she was trying to hide the lipstick you found when the car stopped, she would have insisted on looking for your make up herself. The lipstick could be hers, but it could be equally belong to friend or acquaintance she gave a lift to at some time or, as you've suggested, to the previous owner of the car.
You seem so fixated on the possibility of your dm having an affair that you've ignored the possibility that her jealously when a certain woman calls your df may indicate that he's the one who's unfaithful.
In any event, it seems to me that you would benefit from counselling to address the way in which your apparently dysfunctional relationship with your dm is adversely affecting you to the extent that you're continuing to go over old ground and find cause to support your contention that your dm is having an affair even though you no longer live at home.
The bottom line is that your dps' relationship is entirely their own business, just as any adult relationships you enter into are no business of theirs. By all means work towards having a better relationship with your dm, but put your suspicions to one side before attempting to achieve any improvement in the way you relate to her.