Have we any clue that such a exchange in the theoretical OP strength between two channelers could happen?

I mean, we know that channelers can in the worst case burn out, if they mess with *angreal. Therefore I can buy that maybe Moiraine burned out and Lanfear was at least temporarily unable to channel, as a side-effect of falling with another channeler through a melting ter'angreal while both were using an angreal. It also makes sense that Moiraine actually wanted to achieve that, because otherwise Lanfear could likely simply return to Randland and attack Rand again.

But it seems pretty crazy to me that this incident could somehow lead to Lanfear (why not Moiraine?) losing an - apparently completely random - amount of her possible OP strength and Moiraine (why not Lanfear?) gaining this potential, especially since according to BS the OP strength is tied to the respective body of a channeler (or do you think that Lanfear and Moiraine changed bodies?). That such a thing could happen and would even have an important plot relevance seems like a complete deus ex machina explanation.

Another problem is then the question, how the Finn can hold Moiraine, if she is such a strong channeler now (though I suppose one could also explain it with some super ter'angreal they have).

Finally, in her last PoV in TFoH, ch.53, Moiraine suppresses "a small bubble of hope", before she attacks Lanfear. As we learn from her letter in KoD, Moiraine definitely knew that she would survive this, but would be captured by the Finn with the good possibility that Thom & Co. will successfully rescue her. Therefore I think the small bubble of hope Moiraine suppresses most likely means that she hopes against all odds that she won't burn out, though she is very certain that this will happen.