1) I would think that IF each knows each other AND have a very trusting bond, both will remain silent and take the 6 months. Otherwise, they would each betray the other an assure that they do not get the maximum penalty, and possibly will get off entirely.
2) The choice seems to be between not trusting the billionaire about getting the money and/or the effects of the toxin, and thus not promising intent to drink the toxin or...
...trusting the billionaire. Then to honestly get the money one must promise intent to drink the toxin and then to actually drink it. One can not honestly promise intent knowing that you are later going to withdraw said intent. Of course, I guess you could truly honestly promise and then later rethink it and refuse to drink the toxin. Early idealism followed by later pragmatism.
Personally, I think I would decline the offer.
Moose