Indeed it's not. The "mistake" originates from RJ's notes, Harriet and co. merely copied it by accident. In her main entry, Cadsuane's strength is given as 5. Almost everywhere else, including in a note where he compares her level to Egwene's and Elayne's potential, she's a level below their potential, so 9.

We could argue to death about the reason for the discrepancy - for instance is it a mistake or a change of mind along the way, but personally I believe it's just a mistake. I believe RJ when he wrote down Cadsuane's entry and strength intended her to be as strong as possible without contradicting the books, so he placed Cadsuane right below Nynaeve's current strength, forgetting Egwene/Elayne too were described in the earliest books as having higher potential than anyone in the last 1000 years.

In all documents, those RJ kept as quick reference to write, that was properly corrected to level 9, so I think he realized (or this got pointed out by his assistant) his mistake pretty fast, at a time when nothing had even been said in the books about Cadsuane's strength.

Siuan's "after" strength in the Companion matches the notes where it appears a few times in documents from different years/books. This makes me seriously doubt it varied over the years. I think you guys simply tend to overestimate the precision of some of those AS comments. Not every one of those comments, especially when they're just thoughts, is purely objective and untainted by emotion or other reasons like discouragement, self-pity, tendencies to boast etc. etc. Nynaeve is a prime example of bad faith in some situations, and she isn't the only unreliable narrator RJ used.

Moiraine's strength post-Finn was established by RJ himself. It appears in a single note that establishes those facts and is likely from the time of KOD, or written in preparation for AMOL. The same note also mentions how strong her angreal will make her. There's no reason to believe in a mistake, this was a late series note when the system was fully in place. The Finns drank Moiraine's ability to the brink of death.

There may be a few more mistakes in the Companion about strengths (and a few debatable decisions when RJ had a choice/range of possible strengths for a character - not sure what they did in those cases.. choose or simply not give the strength) but as far as I can tell, 90% are exactly those in the notes, where aside from Cadsuane and a few minor things the levels given are very consistent from document to document, and as those were the files RJ used while writing, if they appear to disagree with the books there is most likely a reason(like intentional mislead/unreliable narrator) or error of interpretation from the readers.

Half the series? Keep in mind there's absolutely no solid proof of that. The two notes where this appears aren't dated (and one is about other stuff and just refers to Demandred as Demandred/Taim), and it's extremely difficult to do so with any certainty. Personally I believe he wrote them at the time of TFOH. I'm fairly convinced the only published scene in which Taim was Demandred in RJ's mind was the one at the end of TFOH, and by the time he wrote LOC, the final draft of it anyway, I think RJ had changed his mind and already decided that Demandred would rather stay in the shadows and control Mazrim Taim, his agent, while Osan'gar would be the Forsaken sent by the DO/Moridin to keep an eye on Taim (it's overkill to send Osan'gar to keep an eye on Demandred...).

Terez believes it endured much longer and he changed his mind maybe as late TPOD, but she's convinced that RJ changed his mind because too many people figured it out, but I find that argument totally absurd since if RJ was an expert at hiding what he wanted to hide, and in this case a connection of some kind between Demandred and the BT was pretty obvious right away, which means he intended us to suspect a link.

He also wasn't in the habit of lying blatantly in Q&A, and I see no good reason to believe he did about Taim/Demandred, which to me means the most likely scenario is that Taimandred was discarded as early as LOC.