You pay immediately at the point of application, there's no option to pay later.
It's a bit painful but there are applications that allow you to put multiple jpg-s to one page. For additional document submission, there were new upload fields added to the system - email they sent me referred to those fields. I guess these are customised as per each application (if / when additional paper is needed).
Tbh the instructions were extremely clear. I spent the most time on getting multiple jpg-s to one file. Ended up using Canva.
i assume they check most of these themselves as I only submitted the docs listed in my original post. I think they require the trainer to have relevant certification (point 4).
I guess depends. It will be annoying carrying that printout every time (I assume soft copy will work) but I don’t want to give up a full page for a sticker in my already full passport that has 3 years until expiry.