I think the 35% fail rate for t3/4 (before s2) is the more important number. I you don't buy your familiar until t5, it'll sit out shuffle 2, and won't come back until s3, typically t7 or later. That's a BIG set-back.
FYI, that's bigger than the odds of two terminal actions colliding in t3/4. If you don't feel comfortable opening with two terminals, then you should only open Potion-Silver if there's some good consolation buys for your $2+P.
The logic here is not quite complete. The *amount* you benefit/are hurt by getting the right draw matters. Sometimes you open with 2 terminals if they are good enough (particularly when one of them is chapel), because there is a major benefit if you do happen to draw them separately, and drawing some terminal with the chapel is no worse than drawing silver with chapel. But you would probably not want to open something like moneylender/swindler, since the benefit of getting to play both does not outweigh the damage resulting from drawing them together (compared to moneylender/silver for example).
Similarly, when familiar is the only curse-giver on the table, you probably have to take the risk of getting the potion right away, because doing something else is just as likely to lead to a loss as buying potion and missing. Existence of $2+P is little consolation, as if you get a familiar and I get a scrying pool, I lose anyway.
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