How do I trigger next scene in Ableton?
When you are in MIDI map mode, you will see extra buttons under the scene section for mapping the movement between scenes (up and down) and launching the selected scene. Also in Preference >> Record-Warp-Launch, you can choose to "select next scene on launch" if that is what you want.
How do I play all scenes in Ableton?
Select all the tracks at the top from left to right and press Ctrl+G to group them. This creates a group of all the selected tracks and now you can setup a follow action to play the next scene for each scene on the grouped tracks.