That one is a fact. Comey was directly responsible for the making sure a special prosecutor was appointed by releasing his memo to his proffessor friend (who we later learned was also on the payroll for the FBI). And that happened after he was fired. So that part IS a fact.
Could it also be argued that it is a fact that Trump admitting to Lester Holt that he fired Comey because of this Russia thing, is what “sparked the Mueller investigation?” Or in both cases, are these just opinions of what may have actually “sparked the Mueller investigation?”
No, “sparked the Mueller investigation” is not a fact. You’re conflating two separate issues: the already in progress FBI investigation, and the appointment of Mueller to take over that ongoing investigation. So firing Comey didn’t “spark” an investigation.
Sure. But just because it was his hope, does not mean that is actually what was the spark for the investigation, In your opinion it may be. In my opinion it may be his interview with Lester Holt. In the opinion of @Airyaman it may be the conversation he had with Russians the day following Comey’s firing. In a thread asking for facts and not opinions, it doesn’t make much sense to allow some opinions to be presented as facts, and not others.