My understanding is that there is no way Moridin (even if assisted) could balefire things fast enough to overcome the Pattern's self-healing. Moreover, the results of BF aren't necessarily the "destruction of everything" - for all we know, the Pattern could respond by weaving completely new rules, items and people into itself. So it would make the world more like T'A'R. This is not really what the DO/Moridin want, I think.

Rand may have only hit 300 odd people, but the size of the BF stream was probably similar to what was used upon cities in the War of Power, if not greater (as this is the first time we know of that the CK has been used to create BF, and we have the (arguably) strongest channeler in Randland using it). The fact that this damage occurs to the Pattern all at once is the real danger.