Ladies and Gentlemen: This article describes the planche lean. A similar shape is used for planks, which is very different from a "normal plank" where the shoulders are retracted. Do not confuse the two. What is shown below is a specific developmental exercise that is fundamental to getting your body ready for planche work. This is why you need to make sure you concentrate on the body shape! The difference between plank and planche lean is that you are protracting as hard as you can with a hollow body and straight arms but not leaning at all into the plank. In the planche lean you are both protracting hard while hollow AND leaning forward with straight arms. That's why they have different names. You need to work on the plank first, so keep that in mind after you are finished reading.
A planche is all about the lean. I see many people attempt to "push up" into a planche. This is not correct. Even with tuck planches you should not be thinking about trying to lift your feet off the ground. Try this: Start in a tuck planche with your feet still on the ground. You should be as hollow as possible. The more hollow the better. If you don't know what hollow means, think about trying to touch your shoulders together in front of you. Slowly lean forward. At a certain point you will notice your feet begin to peel off the ground, at that point simply lift your toes. You are now in a correct tuck planche. Some of you who could do tuck planches, may notice you can not hold this position. That just means you were lifting into your planche and never strengthening the lean. This is also the reason many people have trouble moving on to advanced tuck. Here are some pictures of correct body position:
I'm a little crooked but you get the idea.
Correct Planche Lean
To answer your questions, build up to 30s holds on the planche leans. This should be done on both floor and rings. It's very easy to lean farther and farther as you get stronger. Try not to do this. Pick an amount of lean you are capable of and follow SSC cycles to increase that lean. Again the key is to lean, not lift.
I don't know the answer to that as I have never done them that way. However, Gregor recommends to perform them that way so it should be taken into consideration. It's up to you.
Naterman Edit: Leans are much harder with feet elevated to the same height as your shoulders. You should probably take the time to develop a good lean with feet on the ground before attempting the same thing with your feet elevated.