On 2 and 3: aspartame metabolizes into the amino acid phenylalanine, a precursor to dopamine and epinephrine[0]. In normal people, breakdown of the upstream neurotransmitters increase when phenylalanine increases, meaning there are no negative effects from it's consumption. It does not store in the liver.

There are some diseases that involve a malfunction in phenylalanine processing, like phenylketonuria[1] and hyperphenylalaninemia[2]. It would not surprise me a bit if people with subclinical forms of hyperphenylalaninemia self-medicate with diet soda to feel better, particularly energy drinks that come with other things that aid in dopamine synthesis. Disfunction in dopamine synthesis often affects other things like NO synthesis, affecting blood pressure and inflammation.

[0] https://en.wikipedia.org/wiki/Aspartame?wprov=sfla1

[1] https://en.wikipedia.org/wiki/Phenylketonuria?wprov=sfla1

[2] https://en.wikipedia.org/wiki/Hyperphenylalaninemia?wprov=sf...

Edit: changed serotonin to epinephrine, as phenylalanine is not a precursor to serotonin.