1. I agree that BS obviously forgot about Graendal's placing of Cyndane above her.

I just went back and re-read the quotes where BS discussed thinking Cyndane was very weak. Here's the first part of the quote:

I did not think that Cyndane should be nearly as powerful as she was put in the books as being, so I had been under the belief that the Dark One was pulling shenanigans…

Note that he acknowledges that Cyndane was depicted as powerful in the books (including, presumably, that she is stronger than Graendal). He just thought that her strength was being enhanced by the DO (and, presumably, that Graendal detected her enhanced strength), and that her natural strength (which Graendal couldn't detect) was very weak. Here's the next part of that Q&A:

Matt: ...like a little, in essence, let’s say what the Forsaken Lanfear did to Asmodean, you thought maybe the Dark One was doing some similar…shielding…
Brandon: …or the other way around…here is a little extra Power you can draw upon while I am pleased with you, I can take this away…
Matt: ...that is a question…let’s jump to that question, there have been some theories that talk about Lanfear…
Brandon: ...let’s back up and say I was wrong. I was interpreting the notes a certain way. Maria was able to pull something out that I had not seen that made it clear that I had misinterpreted, and that that is not the case, Cyndane is not under any shenanigans. What you see is what you get.

That doesn't disprove your theory - BS could have thought that Cyndane's natural strength was very weak for the reason you suggest. I just thought it was worth clarifying.