AST's weakness in end game

[AzureFlare]

It's never a waste. NEVER. It could become a problem if you are running low and using every Ewer with Royal road
Of course, it could be better on another things but it will not be a waste.

Again, as I said before, it'll be inferior unsyncing CO and LA just for a 5 second Balance extension if you can get another round of DoTs out which you couldn't before. If you don't need the mana or have the globals to spend, sure, unsyncing will be in your benefit. Including normalised missrates and using the mana on just Malefic/Malefic 2, you'd need like 7425 raidDPS for it to be more mana-efficient than waiting for your first LA cast. In a burst session, sure, that's entirely possible, but I doubt that window lasts longer than 30 seconds which your regular expanded Balance already covers.

If you can fill all your globals without help of a BRD/MCH then of course it goes way deeper than that and using blanket LA will be fine if not better, but in fights like A2S or higher floors where bards/mchs lose DPS on giving mana thanks to large amounts of uptime, I have a hard time seeing that happen as opposed to mechanics-heavy, mana-slumberfest Thordan.

Theorycrafting

It is possible to fit 5 Lightspeed with Diurnal's boost, without it you'll need some spellspeed. Given, that does include Diurnal's speedboost. CU and Disable were mentioned, though I'd only count for Disable on critical moments as normal Virus would be close near a non-burst/buster Disable. Also didn't account for E4E on WHM to balance on that account, so CU would be the most noteworthy.
The 2 ticks from LA + CO are pretty much guaranteed as far as I know. It feels like the ticks are slightly faster than 3 seconds anyway, fast enough to guarantee 7 ticks in 20 seconds even if the first tick comes very late. (Edit: Just checked it. If the natural MP regen tick is 3 seconds, then LA definitely is faster. The first LA tick came after it and near the end, they synced up).
Honestly, we could discuss more points but I feel it becomes too contextual once we go deeper. Nicely done.