Well, agreed, even with a quite short charge time you can get a spark when you
break the current in the primary side of the ignition coil. But therein lies the rub: you only get a spark when you
break the current, not when the current
starts. So to get that spark, which it needs to run backwards, the points need to
lift near TDC when running
backwards.
The OP's short opening time causes the points to close 2.5-3.0mm
aTDC when turning in the normal direction, so when running backwards they open 2.5-3.0mm
bTDC. Which is a fine timing for running! So it can run backwards, enigma solved.
Now
why it has such a short opening time is another question. If the cam is standard & fitted properly, it has to be because the end of the rotor, on which the cam is fitted, is out of true. This is possible if the rotor is fitted poorly, or if the shaft (=the end of the crankshaft) is out of true. For a 250 single, crankshaft imbalance is typically a max of 0.02 - 0.03mm, which you would also see at the end of the shaft, when the lot is fitted in the main bearings. Now the end of the rotor is further sideways, and that will amplify that number (by the ratio of distances to the bearings). Say typically 0.05mm max at the end of the rotor? Which you can measure at the end of the collector.
Measuring run-out on collector (dial not zero'ed).jpg
For a better measurement you'd have to fit the dial (which was not zeroed in the pic) to the engine, rather than in a stand as I did here (easier).
Measure not only the run-out, but also the direction in which it deviates wrt the location of the points' cam follower - that should explain the observed lift profile.
(Without a dial indicator, make a solidly fixed pin close to the same place & measure with feelers.)
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