Regarding Alchemical Past Lives: It is explicitly stated. The Alchemical is not a continuation of their prior lives, they are an amalgamation of them. There are missing parts. If I took your memory of graduating kindergarten, my memory of my 7th birthday, and Gensh's memory of watching a basketball game the result would not be any of us, it would be a separate being assembled from our memories. The proper continuity is no longer present. The issue is not that memories are gone, it is that it is no longer the same person with memory as one of the big reasons.
An unheroic soul is NOT the same thing as unable to bond to a celestial exaltation. That is the entire point of the other Solaroids. Infernals explicitly failed, by their own choice or circumstance, and Abyssals bargained away the entire world so they could keep on living, often out of fear.
The point of distinction between Alchemical heroism and celestial heroism is that Alchemicals cannot claim that the heroism that catalyzed them is their own. It was cultivated over generated by maneuvering other people in positions where they would perform the proper activities. It would be like saying you are rich because of hard work (Exalted because of heroism) when a bunch of other people who did actually work hard died and left it to you. This doesn't mean you can't continue the trend, but you didn't personally earn it.
Also, the process of cultivation seems to be designed to reduce uncertainty in Champions. The result being that their proclivities make it more difficult for them to make independent choices contrary to their past life conditioning. As I said, like breeding submissive people together to make better slaves.
Finally, terrestrials. Given that there was a time in which they all Exalted, it seems likely that they also don't have a choice in the matter. Every single offspring, regardless of environment, will exhibit certain behaviors. This implies that they will never have a choice in the matter, the inbuilt programming with force them to behave "heroically". However, this implies that it isn't actually heroism in the same way courage is impossible without fear.