I agree with you that Callandor could be one of the keys to sealing the Bore. I never really thought Callandor would play a big part in the Last Battle besides someone like Narishma using it to kill a bunch of Trollocs and Fades. Well, that is until the last book when Min kept mentioning Callandor and how she thinks the prophecies about it aren't as simple as Rand thinks.

My theory revolves around Rand still having access to the True Power, which in essence is the Dark One. If Rand could channel copius amounts of the True Power through Callandor, this would weaken the Dark One I assume, and he would lose his "grip" on the Bore and then the Pattern would be able to heal itself and the Bore would no longer exist.

Another option, if Rand does not have access to the TP, is Rand allows Moridin (or switches bodies with him, a popular theory right now) to "steal" Callandor from him and Moridin begins channeling the TP through it. We know that the TP becomes highly addictive the more one uses it. If channeling the TP using 100% of Callandor's capability, Moridin would be strongly addicted immediately and would not be able to stop. Unwittingly, he would be draining the Dark One of his power.

I do like your idea that Fain will come in to play. He hasn't been killed off for a reason. His interaction with a weakened Dark One or with the True Power could cause a backlash which caused the Dark One to lose his grip on the Bore.