The descriptions are okay. One needs to keep in mind the card text is a compromise between clarity and brevity (clear and short). It also needs to be translated into 12 other non-English languages. Part of the learning experience about a card is playing it.
Expendable Performers does not require exactly 7 summoned Illidari Initiates to die. If it summons only 6 or less and they all die, the second summoning effect will proceed regardless.
https://hearthstone.gamepedia.com/Expendable_Performers
Reference: https://twitter.com/Celestalon/status/1324001570032242696
As for Magtheridon, it does require 3 to die to acitive. The summoned Hellfire Warder states, Magtheridon will destroy all minions and awaken after 3 Warders die.
https://hearthstone.gamepedia.com/Magtheridon
If you want to provide feedback / suggestions on card text, it would be more appropriate in a discussion forum (community section). There the discussion can happen to see the scope and impact with a larger number of players.