Sooo... that doesn't answer most of problems though.
The mages that casted summon imps where halfway across the battlefield. Where is far enough ? Since abyssian mages burn slaves faster when they are near each other it's a bad idea to group them, and you need to put them apart from each other if you want to have any slaves left to cast any spell. Isn't there another way to prevent them from casting spells pointlessly ?
Yes, abyssian mages tend to burn slaves. Litterally. For that reason too waiting another turn to use reinvigoration is kind of a suicide since the slaves start catching fire about turn 2... That probably works with other nations though.
Slave matrix and master device cost a lot of earth magic and weren't available in my case since it's pretty much in the beginning of the game. I will think about it for later though.
I quite agree that blood magic is infrequent, but I think it's rather due to the fact that the slaves get killed by battlefield wide spells rather than anything else. The blood spells by themselves have pretty amazing stats compared to the non-blood equivalent. I therefore planned to use it before that.
Well. The conclusion seem to be that I should have used communion instead, and that sabbath is not cost effective in its current form.