That should be right(and everything else you posted is spot on), but for whatever reason, the output does increase somewhat at higher voltage. I don't know why, or have an explanation for it, but it's visible in person and we can see it on the graphs that chart brightness along with runtime. The difference isn't enough to matter in actual use(IMO), and I've never cared enough to pursue an explanation, either, but have always been a bit curious as to why.If I remember correctly, when you increase voltage with this driver it will just lower the current draw from your cells. The driver does not allow any additional current to be delivered to the LED. The question is, will that 18650 drop from 4.2v's down below 3.4v into direct drive before two cr123's will drop from 6v's to below 3.4v and that all depends on your batteries capacity. The output should not change (at least nothing you could physically see) as long as whatever cell you are using is above 3.4v's and below 9v's.
Anyhoo, due to the greatly increased capacity vs. 2 shorter li-ions, 1x18650 is certainly the most practical power source for the M61 in a MD2 body, in spite of not being designed or optimized for it(hence the long taper with gradual dimming vs. full brightness 'til the battery is exhausted). That's what I've always used, but there's a downside-for me, anyway.
My take is that if I want a M61, I want a M61, not a M61 that becomes a M61L halfway through the battery life, then a LL, LLL, LLLL, afterwards. Personally, if a LL was what I needed, that's what I'd have been using to begin with, because it would give steady brightness for a lot longer.
Which is "better" always depends on what you want from the light, but I actually swap batteries less often with the M61HOT in my work light than the M61N it replaced, because it has longer regulated runtime(which is what I want from that particular light) on 1x18650 than a regular M61 with a buck circuit.
There's a big difference between "designed for" and "can be used with", and that somehow escapes a lot of people.