Assuming we did know about their plans, and that those plans were counter to our desires. What could we do about it? We can't go to grandma without tipping her off, and going to dad will just make it harder to eventually unseal Jaffur.
I'd argue that telling Berra about the Ancestor Cult wanting to unseal Jaffur would actually make it easier for
us to unseal him, as it'd convince him that we're on his and Dandeer's side. Him and/or grandma being angry about us infiltrating the council is still a problem, but, as I said earlier, we could pass it off as a vision or ask Sensei to do the same.
Secondly, there's plenty of other bad things they could be plotting in there that'd be far easier to tell Berra about, such as a plot to excommunicate Dandeer, burn her for heresy and then put Clan Vegeta under theocracy (the last thing's not that probable, but them killing Dandeer is certainly a possibility).
And if they do decide to unseal Jaffur? Good luck getting past Berra without us, which means they'll have to bring us into their plans anyway and we'll be able to tell them about Jaron then.
They don't, actually. Firstly, they can just grab Instant Transmission in, grab Jaron/Jaffur while he's alone, Instant Transmission out, and then unseal him. No fight with Berra needed.
Secondly, Apra's a member, and while she's currently somewhat weaker than Berra, she can still at least stall him, and that's if she doesn't train herself back up to her prime in preparation to whatever they're plotting.
Thirdly, if they really wanted a second Super Saiyan for any sort of plot, I think there's a much higher chance of them involving Yammar than us, as Apra would
absolutely not them to involve and therefore endanger us.
I still have one cookie left so if a plan with the infiltration wins, I would place it there. However, both the zenzu and multiform are worthy of it too. So right now I am trying to decide whether to go full ninja or not.
Currently I get the feeling that the infiltration wouldn't just be a single roll but consist of multiple story updates (if we're lucky it could give us a trait), so I don't think we can spend cookies on it beforehand.