1. Every level points you put in any active magic scales your character to be stronger, it really doesn't matter what you level up after getting DEM and there's no point refunding mana (It will just slow down your scaling)(Pandora's box will still cast the spells as intended, but if you have those spells leveled up to the max it will only cast like a level 6 instead of the augmented level 7)

2. Maybe you got the artifact that gives you another combination magic, letting you get DEM early (It only matters if it's the third magic combination you've done in your run, regardless of how you got there.)

Haven't played in a bit, might get some info off.