Well, I kind of understand the Chrysalis one; but I disagree with your point on Spike.
I mean, what Zecora said is that it's in a dragon's nature to be greedy and lazy; but she never said anything about it making him grow naturally. I actually believe piling up a bunch of things like the adult dragons we saw in the show just accelerates their aging process and locks them at adulthood if they keep it for too long. Spike would just age normally with time, since he accepts the fact that greed makes him a monster. Maybe their age proportions are big compared to ponies, so a dragon that's 12-ish is still a baby; because teenagers are around 80 years old. Stuff like that.
Also, I believe that Spike becoming an irrational adult dragon is a metaphor for what greed makes out of people who can't control it. I think that was the point of the whole episode.
As for the Dragon Quest part, it's been made clear that Spike doesn't want to be like the other dragons himself; and that Twilight was just worrying for his safety.