Any question answered...

Yes.

Longer cranks only work if you can move your knee over the required distance both comfortably and while making optimum power. To do that, you need either a long femur or a large range of hip flexion, or some mix of the two. Of course, long femurs usually go with tall riders, who usually go with large frames.

FWIW (i.e. nothing) I have 175 on my geared bikes and 170 on my singlespeed/fixed bikes, with the exception of my TT bike which has 175 at the moment and will have 177.5 soon. I can't say I ever notice any difference switching from 170 to 175 or vice versa

Is there a method for deriving a persons optimal crank length from simple measurements, easily obtained with a tape measure?

I found one on-line which simply took your inside leg measurement (34") and spat out a result of 172.95mm, but that seemed a tad facile.