First off, thanks for the long answer. I do not share all your points of view, but I appreciate the effort you take to answer.
I know that some endgame players can 1 key Nightmare Hydra....I even know how they do it. One of the possible answers is a +4 Trundra hitting the decapitated Head of Torment. I can assure you, that I have enough knowledge about the Hydra Heads to 1- key Normal and Hard Hydra (my clan has not advanced to brutal).
Stil the time needed to do this is way too long and it includes several times of restarting to get the correct heads regrowing even on normal or hard. I am happy you included some links for CCs videos which are very helpful. They underline your position about the possible team compositions very well...and they underline my position of the RNG overload. MurderInc says himself, that you should probably reset your entire key if you are getting the head of decay in the first regrowth (~8:45). For me, this is not a strategy to work against this head...this is just pure admission that the RNG factor beats any strategy. But we might differ in this opinion.
For me, it does not sound like a challenging content, if the solution to a challenge appearing is: Restart your key, hope for better RNG. But as I said..we might differ in this...
Regarding Mithrala and the future announcement of changes....sorry, but I have heard so many vague announcements of Plarium concerning features like QoL improvements of tavern (which have not been implemented yet), about bombs or hexes being viable (not at the moment) about champions getting not crazily overpowered (which a team of four +4s is in my opinion) or explanations that are basically insulting (after more than a year we give you a mediocre legendary and a trash epic instead of 6 sacreds and 6 5*-chickens so you can understand the Hex-mechanic [the normal player understands a 4-lined text block about Hex and does not need to play for more than a year to understand this mechanic]) that those announcements have lost a lot of credibility for me. And even announcing the changes obviously do not guarantee an implementation in the game (tavern upgrades, reduce time to need to play etc...).
So, thanks again for the long answer. I appreciate if you give some detailed info like the videos you were referring to, as they give details which can be discussed, but it is very difficult to discuss based on some vague future announcements nobody knows yet. I think a good example here is the Clan Shop. Plarium did announce that the Yannica fragments will not be replaced by another champ...they will be replaced by 4* glyphs. This is a concrete announcement and I think 95% of the players are very disappointed about this...
A statement that something will be announced in the future...just leaves no room for a discussion and ends it.