State of the Loop provides visual feedback on whether your are playing or recording a loop, or if the looper is stopped or empty, and this for an unlimited number of loopers.
With standard controllers, you are left in the dark, and you're always limited to a certain amount of loopers depending on the number of buttons it has.

When you work with several loopers in Live, you always have to look at the computer screen at some point, or grab the mouse to display a Looper. Not very convenient for a guitar player !
Plus the expression pedal inputs enable you to control an almost unlimited number of parameters conveniently with only 2 pedals.

Not to mention the fact that you can display and launch Scenes.
So there are actually a lot of differences !

Yes, in 3/4, only 3 LEDs will light up, and for signatures above 4/4, a new cycle will start.

Unfortunately no.

For now I have tried it successfully with
- M-Audio EX-P
- Zoom FP02M
This one didn't work :
- Lead Foot LFX-1

In the presentation video, it shows 2 Loopers next to each other, but that was just for the convenience of showing what's happening.
Of course Loopers can be on different tracks !

Yes, this is totally feasible.
SOTL has a "standard" MIDI mode, where the 5 footswitches behave as "normal" MIDI buttons. ie, you can map them to whatever you want in Live.
You can map them to record and play buttons of a track and record clips.
Alternatively, I'm going to create another device (or maybe add the feature to the existing ones), that will allow you to select the track you want to record on with the [+] and [-] buttons, and start recording on the first available slot with the Rec button. This Mode will replace the MIDI mode (and will be optional).

When holding the [-] button, or when connecting your device without using the MaxForLive devices, SOTL will act as a standard MIDI controller. You'll be able to map the 5 buttons to any parameter you want in Live using the MIDI assignments.
You can for instance use it to record clips in Session view, or to clear all the Loopers at once. That depends on what you need !