I don't believe the rule of thumb one - a rule of thumb measure or solution is where you judge that near enough is good enough. So it's the sort of solution you'd get if instead of using an accurate measuring device you counted off thumb-widths or thumb-lengths in the same way that cubits, spans, ells and hands are all standardised versions of old "body parts" measurements that would have been individual to each craftsman.
Golf originated in the Dutch word kolf. This is a different game but with distinct similarities. Even the kliek, the club used in the Dutch game gave its name to the cleek, the ancestor of the 1 iron. Golf originated on the (fairly) flat "links" areas bordering on the Forth estuary, areas in regular touch with the Low Countries all through the Middle Ages and having many families who had members in both Scotland and Flanders.
Ps and Qs has got several versions. My personal favourite is that it resulted from children being told to "mind" (i.e. remember) their Please and Thank Yous when visiting other peoples' homes. I don't think there's a "true" answer out there for this one.
It's hard to whistle if your mouth is dry. Wet your whistle is just a way of alluding to this when offering someone a drink.