Difflock - you can engage any time as long as you’re on a loose surface and aren’t spinning a wheel.
Yes to the clutch for shifting between high/low.
There isn’t really a need for the neutral, certainly no need to leave it parked in that. Unless you want to do it to make a theft more awkward.
Shifting between low and high is much easier than the other way round, but both are doable, with the clutch, while moving. The gears don’t have synchromesh so will crunch if you get it wrong.
I’m not sure what you mean with the numbers on your diagram, but you can engage difflock in any of the positions, just not on hard surfaces or with a wheel spinning.
hope that helps!