The average of the two limits you quoted.
Not necessarily, the voltage of alkalines varies much more that NiMH throughout the discharge cycle, and the source resistance is both higher to start with, and increases more during discharge. At 250mA load, the source resistance will be losing you something like 0.13V. I've known duff batteries have a voltage that looks good, but high resistance.
View attachment 719548
View attachment 719549
I get ~35lm/W.
It looks about the right ball park, dunnit.
Well, I don't see where Moon are getting 43 hours from.
Looks like it won't take long though.