If the blades speed up and the current goes up when a gust comes then you will not be stalling badly. If you are stalled then the blades don't speed up significantly and the current doesn't rise with the gusts.

Hard stall will result in almost constant output above a certain level. Slight stall may result in less than the increase that you should be seeing.

You seem to be in a very poor wind area so even the gusts you see may not be a very significant increase in wind speed. The simplest test for stall is to raise the operating voltage and see what happens. If you work at 12v normally and you add an additional 6v battery and the POWER into 18v is higher than into 12v then you are stalled ( current may not be higher but power will, volts x amps). If 24v system add 12v.

If there is stall from the cut in point then it is due to having too low a cut in speed and the best cure is to open the air gap and raise the speed. If it works well in low winds but goes into stall as the wind rises then the best cure is to add resistance in the line. It has negligible effect near cut in but raises the speed in higher winds.

You have a large prop to capture the low wind and I think it extremely unlikely, that you will benefit from series resistance, but if you see an improvement with the added volts then I would raise the cut in speed.

When you are trying to capture something from the lowest possible wind speed then you will need to cut in slower than anyone in a normal wind area. The energy in very low winds is tiny and you need a big prop to do any good. In better wind areas you have to consider the trade off in total energy capture for the gain of a few watt hrs a day from low winds. This is again influenced by how many hours those low winds blow in a day.

The big machine looks to be about 2 ohms stator resistance and the other one about 3 ohms. With cut in at 7 mph ( sorry I cant think in terms of your kph ) you should be coming in at tsr 8.5 ish. That would seem pretty normal. The only reason I can see for your bad stall is either that your claimed cut in speed is wrong (which I suspect is not the case unless you have N50 or better) or what I suspected before and that your prop is not tsr6 but nearer tsr8.

If the cut in is right and tsr is right I can't see why you are stalling so badly.

In any case the one that burnt out was either not stalling like the other or you managed to push it hard enough in stall to burn it out. I suspect that the big one should have survived comfortably at 1000W and the small one at about 750. Sort of 3 year survival should have brought these figures up to 1500W and 1kW.

This is pretty good indication that you didn't furl at any sensible wind speed or you are miles out with your current measurements.

You say they were not controlling power with the tail fully furled. This is suspicious as most machines drop power significantly as the tail goes beyond 70 deg.

There is not a lot of point in guessing at this stage, when you can get them down, measure those offsets. Also if there is enough of the props remaining can you double check those angles ( 3 deg at tip 6 deg at half radius )

I still cant see why stall is so bad but the burn out certainly wasn't caused by stall that was caused by output beyond the stator's capability. I assume if it was that badly burnt you melted the solder connections and went open circuit.

Without re magnetising I think the magnets are not going to be any good. Unless you have new ones to compare them with you had better assume the worst or try a test coil and we can deduce from that how much flux is left.

Magnetising is an expert job with high energy capacitor discharge equipment and you may struggle to find someone.