Here is a general advice: Get your knowledge from real science, not from bullshit religious hysteria. Point by point.
"Aspartame accounts for over 75 percent of the adverse reactions to food additives reported to the FDA": This is simply unsubstantiated BS. The most common culprit is food color, aspartame doesn't make the list.
"Too much aspartate or glutamate in the brain kills certain neurons by allowing the influx of too much calcium into the cells": This is pure rubbish. If this was true you would have to stay away from apples, meat, oranges, lemons, mothers milk etc, all of which contain one or all of the substances aspartame is metabolized into, and all in much higher quantities than in a diet soda.
"DKP is a byproduct of aspartame metabolism": This is not true, DKP is a result of improperly stored aspartame and aspartame breakdown, not metabolism. If you drank a diet soda where the aspartame was degraded and DKP produced you'd spit it out immediately since the aspartame would no longer be a sweetener. Aspartame broken down doesn't taste sweet, so there would be no sweetness in your diet soda. Also, DKP exists in almost everything you eat. From cereal through processed meat to cheese, milk and coffee. Mostly in higher concentrations than in diet sodas. The average consumption of DKT from all sources is about half a microgram per kilo of bodyweight from all DKP sources. 7 micrograms are considered safe. If you wanted to drink enough (non-sweet tasting) diet soda to consume enough DKP for it to be dangerous to you, you would have drunk so much diet soda that you'd be dead of kidney failure first.