This is similar (though not the same) as what I went through for my hubby.
Advise: apply for an OWP as well as a Post Graduate Work Permit (
http://www.cic.gc.ca/english/study/work-postgrad-how.asp &
https://students.ubc.ca/career/career-resources/working-canada/post-graduation-work-permit) for him at the same time. So long as it was a post-secondary technical program AND full-time, he should be eligible (and for how long, depends on the length of the program; the college should be able to provide some insight). The PGWP usually takes a shorter time than OWP (look at website for estimates). Between July 31st and when the PGWP arrives, have him take time off work if you want to be absolutely safe. Otherwise, he could technically get by on the fact that it's summer and the program doesn't run during summers, so he's allowed to work via the study permit. A heads-up: we didn't use this technicality, because we wanted to play it safe.
Re #1: leave the OWP open-ended... you never know when employer might change so don't restrict him, otherwise you may have to get another OWP and that costs more time & $$$!
Re #2: see explanation above
Re #3: there's no need to explain unless you think he's out of status for some reason?
Re #4: no need, we didn't do it for my hubby and he got PR end of June - bottom line is that they don't care especially since the DLI was left unspecified. So long as it was a post-secondary program, he's good
Re #5: I'd suggest getting a PGWP and not working between July 31st until you get it. Alternately, there is a grace period (I believe) offered to students that's about 1-3 months to leave the country after their studies, but you could be on the safe side and get a visitors visa for him to keep him in status but that means
no working... the OWA has been taking about 4 months from what I can tell on here, but the PGWP is even shorter, so he'll be back in status as a temp worker shortly (within the 1-3 months) with either approach.
Re #6: get him a PGWP as soon as possible
I'm hoping that you guys were careful not to let him exceed
on average 20 hrs/week while he has been working on the study visa! If not, put the job on hold until he has a temp worker status (either OWP or PGWP). There's no sense in risking his immigrant status if they ever dig into stuff like that.
Hope this helps!