I have a hard time seeing Barenziah taking such a back seat and being complicit in the more hairbrained of her son's schemes. But she's an enigma. And if we remember her state of mind at the end of the Real Barenziah, I suppose its possible that she simply gave over to a passive life and filial piety.
I don't see it as much of taking the back seat, as just observing. Watching and learning. Already seems to be Barenziah's style. She's a smart cookie. I kind of have a hard time believing that she didn't know about the assissination attempt before the Nerevarine did.
I also believe that Helseth actually seems to care for his mother (from Daggerfall) and wouldn't completely risk her life in a stupid test.
Edit: I just realized that Oblivion is the only game in the main TES seriTes that Barenziah doesn't play a part. She is a really important TES character, isn't she.