The Dark One himself states that even he cannot undo balefire.

There is no mention of the *strength* of the balefire necessary to make this so... it's the nature of balefire itself that "burns a thread out of the Pattern" and makes it inaccessible for transmigration.

Your theory would require the Dark One to grab Be'lal's soul from his body at some point in time *before* the effect of Moiraine's body reached him. The Dark One would effectively have to know ahead of time A.) that Be'lal was going to be balefired, and B.) roughly how strong said balefire would be, in order to attempt such a maneuver. Furthermore, you introduce the complexity of 'now Be'lal doesn't have a mind/soul for the last 60 seconds of his life... so he's not actively fighting Moiraine/Rand anymore... so there's no need to balefire him... etc'. Instant paradox!

Anyways. The DO merely watching Be'lal would be insufficient, as at the moment balefire touched him it would already be too late to capture his soul - whether by a millimeter or a mile doesn't matter, balefire removed him from some past existence as well as present. So unless the DO wanted to pre-emptively do this - based on foreknowledge Be'lal certainly didn't have, and the DO was unlikely to possess as well tahnks to Moiraine's ingenuity - there's no way to grab Be'lal's soul. And as I described above... that attempt would introduce irreconcilable paradox into the situation.