Flew lots of 206s in parachuting operations for a few years. I reckon option 3 is a pretty good plan...
If you have more than one aircraft, they will tend to lean to different positions but otherwise you should be able to find the max. power mixture position in a climb at full power at your take-off altitude. This is going to be a little richer that the point where the engine starts sounding a little rough. If you set the mixture just rich of this point it should work out fine provided you always give it full power right at the start of the roll to check - most of them will hold full power against the brakes.
For v. short, suggest plenty of flap, but each aircraft varies a lot in what works well. The former float planes will fly much slower as they have huge elevators. Ideally you want Robinson STOL flaps. If you use extra flap don't forget to trim back a bit and try it out on a longer runway first.
Please be careful with all this stuff. Practise as much as you can somewhere long and safe cos you only need to cock it up once !