That's kind of stating the obvious don't you think? How they could possibly know it magnified the effects of the Taint before saidin got tainted?

As for the magnification you're making a series of assumptions. Cadsuane doesn't even know what the True Power is. When she said it magnified the taint, she meant the effects of the taint on a channeller's mind, no more. Thenin an in an answer to a question about that angreal Moridin said he could use, Jordan said that angreal/san agreal made for the OP wouldn't work with the TP (so it was to use it to channel saidin Moridin could have a use for it). The flaw isn't mysterious let alone deliberate, Jordan explained quite well it's shabby manufacturing caused by the war conditions.

Finally, about Rand using the TP again, it sounds absurd that he would (remember the state of mind Rand had to be in to be able to embrace it.... It's the opposite of how he must be to face the DO...haven't you noticed how Rand's power and even its nature is magnified after he embraced the complete opposite of the state he was in when he touched the TP? ) but even more that Shai'tan would let him draw it again anyway, especially so that Rand could fight him. This is about as well reasonned as Lanfear's stupid belief that she could destroy the Creator with the OP. Whatever BS said obliquely not to confirm Shai'tan let Rand access the TP, Jordan flat out said access to the TP depends strictly on the DO letting you.