“the incontestable fact that proof here means “test,” as in the maxim exception proves the rule” — Wikipedia’s article on “Exception that proves the rule”, citing Fowler, traces it to exceptio probat regulam in casibus non exceptis (“the exception confirms the rule in cases not excepted”)

“Indefinites are equivalent to universalls especially where one exception being made, it is plaine that all others are thereby cut off, according to the rule Exceptio figit regulam in [casibus] non exceptis.”

Note the use of figit here. By the time of Coke’s Reports the modern verb has appeared:

“[...] upon which Award of the Exigent [a summons to appear on pain of outlawry], his Administrators brought a Writ of Error; and it was adjudged, That the Writ of Error did lie, and the reason was, Because that by the Awarding of the Exigent, his Goods and Chattels were forfeited, and of such Awards which tend ad tale grave damnum of the party, a Writ of Error lieth, although the Principal Judgment was never given; in this case, Exceptio probat regulam, & sic de similibus.” (Note also Coke’s use of the pleonasm “reason … because”, frequently denounced nowadays as a neologism.)

Cic. Balb. 12.32 says something similar, but without either of these verbs: “Quod si exceptio facit ne liceat, ubi [non sit exceptum, ibi] necesse est licere.” The evident slip of the pen, tongue, or mind has been repaired in a variety of other ways, but all in the same general sense.

That said, the proof of the pudding is indeed about testing, not about fixing or proving.

I wonder if hobber-dobber is somehow tied to hubba-hubba, in the older sense of encouraging a slow player to get moving. I also wonder if the -er of the one and the -a of the other represent non-rhotic and rhotic spellings of the same thing.

Lastly, I quite agree with you about spelling bees.