AHHHH THANK YOU GUEST YOU ARE MY HERO

Ah, thank you!
I wasnt entirely sure

Blue Dances With Wolves would probably be more accurate.

Same

I don't know if I'd call it insanity. He just ended up in a very dark, bad place emotionally. I also know that he wasn't suicidal.
The role lead to him doing the things that lead to his death.
Though I also don't doubt his role in The Imaginarium of Doctor Parnassus helped that along.

Yeah. It gives them a direct link, which is very necessary with someone as unstable as him, especially if he were to learn of their true parentage. Perry would be there to protect them.

Excellent! So!
Linda and Doofenshmirtz were a canon couple, but Linda decided she just couldn't deal with his rather childish nature and desire to conquer the world any more, especially not when she learned she was pregnant. Not with Phineas, though, but rather Candace. She got out of the relationship, and never revealed that they had a daughter.
Doofenshmirtz married Vanessa's mom very soon after, possibly because he had cheated on Linda with her, and was stuck with a shotgun marriage. This is possibly why he at no point attempts to get back at Linda for hurting him and breaking his heart.
Years later, and he and his wife seperate for unknown reasons. He desperately needs love/comfort, so he reaches back to his ex Linda. She's unmarried, raising Candace well with royalties from her one-hit-wonder time. She reconnects with him very briefly before he and Vanessa's mother reconcile. This gives us Phineas.
This is why Perry stays with them, as opposed to any other family in the area.

We live in an age where the grade you get is more important than the knowledge you acquire.

Same, friend.
Same.