On Dragons being over powered for their CR. The Gem Dragons properly match up, so I can see the other Dragons being fixed come the new core.
I figured out that the MM dragon CRs work out if their breath weapons only hit one target (as opposed to two, which is the default assumption for the DPR calculation for area effects). I think it's likely that they statted up the dragons really early on because, you know, it's in the game's name, and also Kobold Press needed them to get the Tyranny of Dragons adventures out ahead of the core books, and therefore hadn't nailed down the maths yet. The dragons in Fizban's do indeed have the right CR for their damage though, and this leads to weird situations like Greatwyrms having less dangerous breath weapons than their Ancient equivalents!
I made some 'rebalanced' statblocks for the MM dragons, and the rule of thumb is that their breath weapon's average damage (per target) is the same as the DPR from their other attacks (not counting Legendary Actions), plus a little bit of hit point tweaking for the older categories IIRC.
But yeah, the conclusion to draw from all this (if any) is that CR is an estimate. It already assumes the monster is going to be used optimally, cheerfully burns through all its resources in one fight, and conveniently dies after three rounds. You have to understand the system's limits and what they're
for before you start hacking it. In the case of the quickling, it is CR 3 by the numbers, but they probably knocked it down because there are a lot of situations in which a competently-played party can kill it almost instantly. It's the reverse of the ogre, which is technically CR 1, but its DPR is too much for a level 1 party to handle so it's bumped up to CR 2.