The recent nerfs were about healing output (how strong the healing is) and Tyr’s Deliverance, not the mana costs in general (only in PvP).
You can read it up here.
It does. I have it every time I play a Mythic key. In the +18 - +20 range. Not lower keys. I am not playing there anymore. I have moved on from that key level 2 weeks ago.
Define “playing right”. If “playing right” means for you the definition to play Daybreak talent row and nothing else, that is not playing right. That’s playing the Meta. And a game should never be driven by a Meta in such generic endgame content as M+, because it kills variety of play styles. Not even in Mythic Raids it should be required to play a certain way to beat a challenge. That makes an RPG just linear and boring.