The answe to all you questions is 42.
Don't hold me to any of this.
1, I assume that the video you dropped on to the scene will load evey time the scene loads. Then the switch in the code happens.
I would suggets not using the dropped on video but a rectangle with the video tags as I have done. You can still point to the video in the resources.
2, I do not have an Andriod so I cannot test this..
3, The video tag code is standard, there are plenty of examples about on this site and www but you would simply just add multiple sources to the tag and I guess the JS. I have not ever needed to do this with the JS so you may have to get creative with the JS to change all the sources.
4, Not sure on this. There may be a way to fix that, may set a poster image to hold the space while the load happens.