Seeking Software Recommendation
What kind of trainer would I be if I didn't put in a shameless plug here? You should use Mainstage. It's $30 and we have a 'plug & play' template should so you choose: http://loopcommunity.com/content/mainstage-template
Instructional video here: http://loopcommunity.com/training/using-mainstage-to-run-loops
1. You can only play audio loops with the Playback plugin. You can play prerecorded MIDI with the Ultrabeat plugin. You can of course play any software instrument in MS live with a MIDI controller
2. You can assign any number of buttons/ faders to any amount of MIDI controllers. The workflow that the video shows involves one PLAY/STOP button that plays from the top and allows you to jump around. You could, if you prefer, create a separate button for every part of the song and map the buttons to a controller to achieve the Live workflow that some people use (ie: jump from chorus to verse to intro to end).
3/4. People are using the EXS24 Sampler Instrument in MS to assign a pad note to each key of a major scale. Then, you can just hit the corresponding MIDI note to launch the pad in the right key and transition. For actual loop playback, you would need to still manually change patches (songs) to the next song.