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I'm pretty sure that min/maxing makes a maxed-out Grand Convocation possible. The question is, how viable would something like that be?
The cheapest options for each slot (to my knowledge, at least,) are:
HQ - Inquisitors or Priests. Either way, 25ppm. 100pts to max out.
Troops - Battle Sisters are 60ppu, or Scouts are 55ppu. 220 points to get the requirement.
Heavy Support - Retributors at 60ppu. 360 points to max out.
Fast Attack - Either Rhinos or Drop Pods. 35ppm either way, 210 total.
Elites - Henchmen squads from Inquisition. 12ppu. 72 points for the cheapest slot.
Lord of War - Gotta be a Banehammer, at 410 points.

So, absolute minimum we're looking at 1372 points. That leaves only 628 points to swap out squads and give upgrades. Can this be made remotely useful, or are Grand Convocations bound to be used only in Apocalupse and supersized games?

Your LoW could be replaced by a Knight Gallant, saving you 255 points. Also, you seem to have forgotten that the LoW is 0-3, not 0-1 which, using the knight, would bring you to 1937 minimum, pretty much leaving you with a bunch of useless bare bones T3 dudes and dudettes plus a trio of Knights and a handful of transports.

As for if it's useful, well, the easiest way is to take something cheap but useful in every slot and then start cutting out what you can. It's nice that it's possible, but now you have to see if it's practical. Even then, I think that most people would opt for the War Convocation anyway if they really wanted to play around with cantical units.

Inquisitors get a warlord trait, so that's nice. another 30 points for a psycher level wouldn't go astray but that's something nice to have instead of required to make the unit function as anything but a points sink. Sisters with flamer/heavy flamer in a rhino are cheap and effective and it lets us kill 2 birds Slots with one stone. Rets with rending heavy bolters aren't terrible, so make that 100 points each, though you could easily replace this with thunderfires for no net gain in points or whirlwinds if you wanted to go on the cheap. Henchies with triple plasma could be a thing, but then, you're certainly paying for power armour as well and probably a couple of bodies so you don't just have super expensive T3 guys.

I suspect that it could be possible to make a half decent if not overly respectable army at 2500, but that's quite a large game for most people.

Whoops, missed that on the titans. Oh well.

Sadly, it's looking like the best choice for the Convocation is just going to be 'Coteaz, plus whatever', to get lots of army-wide Objective Secured. Zealot is nice, but since much of the army is going to be ATSKNF anyways, the 'Hatred' half of the equation is far, far more important than the 'Fearless' - Even then, you're going to need melee-oriented troops, and since for some terrible reason Blood Angels and Space Wolves don't have access to the convocation (The two Imperial armies that would easily benefit the most from army-wide Hatred,) that bonus really isn't that super great. I don't WANT my Sisters of Battle or Guardsmen charging into close combat, and there's only a few Space Marine units that I really want fighting in hand to hand. (Even those units generally work as part of a formation specific Deathstar, too, so that option is out.)