It doesn't matter. It absolutely does.not.matter. Your master files can be 300, 240, 72, 89, 426, 138265, 9, etc. It's completely irrelevant.
It only matters when sharpening for print, and you'll learn about that in the Sharpening Class.
Excellent.
So yes, do exactly what they say. Give them a Level 11 quality Jpeg file, make sure the resolution is 300ppi, but make NO change to the image size. Send them the full-res file that you camera took.
Oh yeah, very vague. Unfortunately, quite common.
Do you know if the banner is ONLY going to be your photo? Or will it be part of a design of some sort?
Oh boy, these aren't easy.
This is the method you need to use: https://www.damiensymonds.net/preventing-banding-in-backdrops/
Does this look satisfactory?
Now, when you hover your mouse over that massive green area, and look in the very bottom left corner of the screen, it will tell you what kind of file it is, and where.
For example, on mine, I've hovered over a big folder and found it's a set of raw files that Nicole sent me:
Can you tell us what your huge green area is?