The flanges are 2.5" in diameter.
It looks like it should be available, but i would call the store to confirm they have stock.
If you do it yourself, you will need a special drill bit and still be certain you are securing into a wall stud. I installed mine in a fiberglass walled tub, however it can be accomplished
Yes it can, just be certain to attach it to a wall stud
Yes. See detailed response to another question.
Yes, though I’d try to catch a framing member (stud) at one end and use plastic toggling anchors elsewhere.
My son had to get screw holders as the studs were surrounded by wires.
No it is not, I had contacted the manufacturer and they assured it was not required nor did I need to use a sealant around the mounting area.