I had assumed that higher difficulties would only raise the AI's skill, making them survive longer and extend runs, but the truth seems to be more complicated. My route for True Meteo, for instance, is MUCH faster at the highest difficulty than on Easy, but in my usual Any Route path, Jeljel takes twice as long or more to beat on the highest difficulty, ruining the entire run. The AI will die much faster with full columns on high difficulty, just like you will, but it's much more capable on certain planets. The Easy route for any category should probably be different from the Brutally Hard one. Lots more testing is needed here.

The "Clear the screen of Meteos!" mission in Multi Star Trip is bugged: you will sometimes not be given credit for clearing the screen. It seems to happen when enemies send you burnt meteos right as you do it. Keep an eye out for a quick white flash; if you don't see one, you'll have to clear the screen again. Luckily...

Failing a mission in Multi Star Trip doesn't prevent you from facing True Meteo, as long as you can still reach it after getting kicked down to a lower route.

Regarding Clean Air Club vs. Hot & Chillies in Multi Star Trip: my guess is that Clean Air Club has a lower average time, but if you're lucky enough to get Dawndus on Hot & Chillies (the less common of the two), it can be faster. One would think that the fewer planets in a grouping the better, since it means fewer AI to kill, but the narrow board on Freaze makes its AI pretty dang tenacious.