From what's being described in the OP these channelers and their adds are only the first part of the fight and of the things you listed, I'm pretty sure you have pretty much every class covered:

10 Healers (Druids/Pallies/Priests/Shamen)
6 Tanks (Druids/Warriors) + Pallies?
4 Interrupts (Shamen/Rogues/Warriors) + Mages?
5 (Warlocks) + Hunters?

The only classes you're missing there are mages and hunters. Hunters can easily sub in for warlocks in dealing with the adds (kiting/trapping), leaving only one class out of the mix. Mages, who can also interrupt (although not as well as rogues/warriors) and may be better for phase 2 since it is a "fire" fight (pun not intended although I don't really know enough to comment on that. Basically, I don't think that the group composition you listed there is very unreasonable. If anything you could even drop a tank from that list since the OP said they usually killed one of the adds before the 2min timer was up and have that add's tank pick up Mag.

You know I spend a lot of time reading DnT posts and I've seen nothing of the sort. TBH with you most of our time is consumed in SS cavern so we haven't even much bothered with Magtheridon, let alone declared him a cockblock.

Magtheridon strikes me as how Gurg paints Gruul. Why would you bother to invest the time on him until you have a sufficient number of people ready to complete their Eye attunement (and based on the datamined items the zone/itemization isn't even finished yet).

The cubes become active in phase 2, and I apologise for the assumption of the aforementioned impossibility to kill, I was more looking for a input from other guilds that may have got the step further into controlling the cubes as I know no better place where members of the high end guilds congregate

Honestly how can you call anything a [word removed for fear of aggroing Kabuel] at this stage in the xpack? The expansion has been out for what... a month? Are you telling me that every single person in your guild is geared in the best items available in one month? Have you even have had time to farm all the fire resist gear from the heroics for that many people? Are people still wearing 5man instance blues? I mean come on, this like stepping into BWL in your dungeon 1 set and declaring razorgore to be impossible.

Well, you cant put the FR on the tanks because the add(s) will destroy them. (When you kill one add, the other 4 gain 33% more damage and a 33% casting speed increase. And I damn sure it stacks up)

The cubes do something pretty important to the fight. I dont really want to ruin the "fun" of this encounter, but its not doable without everyone in the raid wearing full kara // gruul // and some serpentshrine loot.

Is there any good reason this thread should continue to exist? So far I've seen a lot of "well I haven't done the fight but maybe you could try X" and "well we don't know for sure how Y works but still it seems impossible."

The main valid point I've seen raised here thus far is Rambar's, which is simply that if Magtheridon is intended to be parallel to Gruul as a t4 endboss, then perhaps his difficulty is unintended. That's far from him necessarily being impossible, though. Give Blizzard some time to tune. Gruul was tested a fair bit on beta, while I don't believe anyone put nearly as much time in Magtheridon, probably because of the FR requirement (right?). It's still very early -- I'm sure if he's overly hard and Blizzard intended for him to go down at around the same time as Gruul, they'll make some tweaks. 25-man raids with TBC gear and mechanics are still new, and I'd rather have these bosses overtuned and tweaked downward than the reverse, especially because they are gateway/key bosses. Unlike past bosses where an easy boss just meant a loot pinata for a couple of weeks until they buffed it, here if they'd made Magtheridon too easy and then buffed him, you'd have groups of people who basically got free TK attunments without being able to kill the intended version. That would be bad.

Anyway, this all seems premature, no?

Aye I guess it could be called premature, but if it means the few guilds that have had a few tries at him get to share a few ideas and theorycraft a bit of the fight I would personally call it planning ahead than premature worry