The True Power is strictly available if the DO lets you access it (it's himself), and there are hints (eg: what happened to Moridin at Shadar Logoth), that the DO retains some control over what it does, that he can make one of your weave "mutate" to produce something else altogether.

It would be very surprising that Ran uses the TP willingly again. It was a major failure on his part to have embraced it once in the first place - it nearly made him fall to the Shadow in the end. The "miracles" he does since his epiphany he used to make for the Shadow after touching the TP (remember his dark aura... the increased spoilage, or compare how his ta'veren effect silenced the whole AS in TOM but failed completely with Tuon when he was dark Rand)

If the TP has a role at SG, it won't be Rand using it, he'd rather turn against the Shadow something it does (DO, SH or Moridin) with the TP. The major problem Rand has to solve is that "a sealing" of the OP needs to touch the TP/Shai'tan, and that was the failure of his previous plan. He needs to come up with something else... either find a way to seal the Bore without using the OP at all, or place between the world and Shai'tan something that might prevent any contact between the OP and the TP. At the moment, the only thing we can surmise can't attack the OP but that the DO would have to retreat from (because the two would annihilate one another if in contact) is the taint of Shadar Logoth. It doesn't mean this is what will be used (in the form of Fain himself, the dagger or the wound in Rand's side), but so far it's the only thing in the series we know can harm Shai'tan.

My hunch is that the key is to use the SL taint to make Shai'tan momentarily lose his hold on the Bore, then with the control gained by the three ta'veren being "as one", the Wheel will be able to seize that instant to eliminate the weakness in the Pattern. I think Callandor is a decoy. Rand must let the Shadow seize it, and bait it with the fact it magnifies not only the OP but also the Do's taint. Then the SL taint (Rand's wound) will come into play, to destroy the wielder of Callandor and during this shock Shai'tan might recoil, retreat, and this is when the extra hold on the Pattern given by the three ta'vere will let the Wheel re-establish reality at SG. When Shai'tan recovers from the blow, it will be too late - the weakness in the Pattern will be gone, and he won't be able to exploit it to re form the Bore to pass through it (because the Bore isn't "real", it's a twisting of reality by the DO, exploiting a global weakness in the Pattern. Lanfear and Beidomon didn't drill the Bore as such, IMO. What they did is thin the pattern further, enough so that a fraction of Shai'tan's power could slip through, enough to twist reality into the Bore).