I find all of the squire-based lines confusing - particularly anything that doesn't specifically have squire with a trasher - but also reasonably useless compared to other things.
I recalculated things from yesterday and am now getting figures agreeing with yours - not sure what I did wrong.
Anyway, here's some other possibilities of interest:
Potion/nothing: 60.606% chance
Potion/copper: 60.101% chance (Obviously this extends to things like squire as well that give you $1)
Potion/(Warehouse/courtyard): 93.434% chance
Then the interesting one - herbalist. Playing where you only return a potion from t3 to t4 if possible bumps your chances up to 62.88%. However, you get very slightly better chances by returning copper if you have a t3 herbalist without potion and with 3 or more coppers. This lifts you up to a 63.51% chance. Furthermore, in either case, there is a 7.72% chance of getting TWO familiars on turns 3-4. So, if you're playing to maximize the number of familiars you have at the end of t4, with herbalist you expect .7123.
Also, if you think about it, oracle is going to be VERY high, as it can do everything moat can plus a little more. Haven't sat down to calculate this, though, as it would be really complicated in some cases (triggering a reshuffle before the end of t4 leads to needing more information, at the least).
Finally, courtyard/potion going for *golem* does not seem at all an unreasonable play.