Yes your 70 turns of #18 would behave exactly as the 36 two in hand #18 connected in series, but except for a dual voltage system it would seem totally pointless. You end up with a less tidy winding with an extra connection and no benefit.
Multiple in hand is normally used when the single strand is too thick or you happen to have something that will replace it by using 2 or more in hand.
In real life except for minor issues there is absolutely no point in doing it for other than practical reasons. I doesn't give you any other benefit.
I think it has already been explained that beyond a certain wire thickness the physical force needed to wind it becomes silly and there is also a point ( probably near 3mm dia) where eddy losses start to be an issue.
There are changes in resistance as you parallel multiple strands but I consider it confusing to look at that here. Your parallel group of in hand wires for a given number of turns will be the same as that of a single wire. You can only gain a lower resistance by paralleling if you can accept the corresponding reduction in turns ( volts).
I have neglected the minor issues of stacking factor where a thinner wire may marginally be better used than a thick one.
I think the issue has largely come about as machines got bigger and winding thick heavy wire becomes difficult. For machines of 48v and under 10ft there is no point unless you happen to have an existing stock of wire.
When you get down to 12v and large props then you would be in the realms of copper strip rather than wire and that would have serious eddy problems. In fact 12v at larger prop sizes presents many problems and may need sectional windings, but the ther problems make it a doubtful proposition.
Your last question about shorting in high winds is a bit complex for a simple answer. You effectively need low resistance, but that alone is not the answer. Unless you have enough powerful magnet material to induce a voltage in the winding sufficient to produce a braking torque exceeding that of the prop when feeding the circuit resistance you will not stop it.
In real life unless you have a powerful alternator you are not going to stop it anyway in a decent wind and the resistance is more likely to be determined by other factors.
Just to add a bit of confusion, if it will not stop on a dead short you may be able to add some extra resistance and increase the load on the prop to pull it of peak power, having got it into stall region it may then stop with a direct short. If you want to avoid confusion forget this last paragraph.
Flux
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