Well I rather thought that most of your questions had been addressed, but I will take a cut at the 3 of them.
1. D (Drive) is clearly for taking off.
D is for taking off, cruising, normal acceleration, normal engine braking, etc.
The key here is the word normal.
2. S (Second) is it more like 1 and 2 in convenient auto
transmission? or there is something else but high RPM?
Ds is like second.
The advantage of "second' was to keep the Engine RP Ms in a narrow, but High band.
The way an engine works, it is most powerful in it's "power band".
The power band is that range of RPMs between Peak Torque and peak Horsepower.
So, if you are on a two lane road and want to pass a slow driver and want to spend as little time in the oncoming lane as possible, put the transmission on Ds BEFORE you start your pass. When it is clear, make your pass. You will complete the pass much faster ( because the engine will be in the power band) than you would in D because in that case, the engine would be at only 1-2K RPMs at the start of the pass.
3. L (Low) for taking off in snow or slippery conditions with
a max engine torque ( available at low RPM)?
Well, aside from L, are you a new snow driver?
The last thing you ever want in the snow, is max engine torque.
And Max engine torque is never available at low RPM, except with a diesel engine.
I really don't know why Nissan included L.
But it "might" be faster 0-20MPH and therefore if you started in L and shifted to Ds at 20MPH you might be faster to 60-70MPH than if you just started in Ds. (Most people accept that 0-60 is faster in Ds than in D)
And of course the INVERSE of acceleration is deceleration which is commonly called engine braking.
So you Will have MORE engine Braking in Ds than in D.
And you will have more engine braking in L than in Ds.
IF you are coming down a mountain (I am talking mountains here. Not hills.), then shifting into Ds could be prudent and L could be used in extreme conditions.
If you ever find your self in Fish Creek Canyon, Arizona, you would be happy to have L just for it's engine braking as you go down thousands of feet, at a very steep angle, around 90 degree and worse corners (at least 1 180 degree) on a hardpack
road, in 120 degree temps.
Hope this helps.
Homer