I would phrase it a little differently: Imps have the ability to steal..., while Ghosts absorb. :)

Imps don't steal because the opponent's mana doesn't decrease. Instead, they accumulate it from nowhere. But Ghosts do steal... :)

By the way, it would be nice to add silver Pegasus. ;)

Increasing the cost of spells can hardly be attributed to spells...

It seems to me...

That's exactly it... ;) I'm not talking about my assumptions, but about a fact that exists in the game. If at least one Blessed Imp is alive at the moment the spell is cast, then the mana gain is equal to 20% of the cost of that spell cast by the opponent... It doesn't matter how many Imps are left.