1~ I would use baby lap to bead on. (Baby section in most stores. Also called lap pad or crib pad. Joann's fabric has it on bolts also.) Look into using leather, suede or a canvas type material for the backing.
2~ You can make it a fully beaded crown type piece or make it a partial. Once you get to the place by the ear where you can't see it in the picture anymore you can add ties that would go back behind the head. Elastic might eventually stretch out too much. Or hidden combs or hair clips would be good too.
3~ Not sure how to measure it.
Measure from your eyebrows up to just above your head?
4~ When I do things like this I start with the outlines then fill in the middle. Like I do the outer edge then pick one of the inside pieces to work on. Fill in that inner part completely before moving on to the next inner part. I hope that makes sense!
The beads look like sequence, clear glass beads? and maybe small pearls? It's very beautiful though!