(1) If i understand correctly and a diploma is something below the level of a university degree, unfortunately not. If you have a university degree you might be able to use it.
(2)If you want to qualify on work experience, it’s tough -and a letter isn’t enough. You need to find your occupation on the o*net website referred to in the instructions for selectees, then (1) see if it matches the criteria for DV (job zone 4 or 5 with SVP >7, and if it does (2) carefully document how you have held this position for at least two years re job description, job duties etc. something you don’t even have a pay slip for is probably not going to be enough to prove this, unfortunately.
(3) no.