I think you’re misunderstanding how it works.
Strongest enemy in picture below is Kobold Knight with yellow/blue mana; also there are 10 yellow and 6 blue gems on the board.
It means that either 10 yellow or 6 blue will be destroyed by Tutanhatmun’s spell - it cannot possibly destroy 44 because there are not that many gems on the board.
The thing is that Magic stat is getting higher than possible gem count on the board, so, basically, it will always destroy less gems than spell description says even if maximum available gems of a certain colour are present on the field. On the other hand, the spell will keep some potency when your magic is reduced and will not go to “destroy 1 gem” instantly.
Yes.
The designers seem to be anti useful Blue Mana gem destroyers. So they wanted this troop to only have the appearance that it can be a useful destroyer.