My school had my loan paperwork for AGES before they certified it. Okay, realistically it was like six weeks or so but I had to hound them to get them to do the certification. I finally had to say, "Look, I'm going to be coming to Exeter in a week. What time on Friday can I stop in at your office to discuss why this is taking so long?" Don't you know, a day later I got an email that they'd completed the certification and sent it to the lender!
I believe the loan money will be sent to the school before the disbursement date. My loan is being disbursed in three installments, on dates requested by the school. Since the loan doesn't cover my tuition, I don't much care when they do it. I'll just have to pay the difference on each of the three dates.
As for doing it before you apply for your visa, that I'm totally guessing on. Since we're required to show proof of funding, but we don't really get any paperwork back, I requested a letter from my university student funding office that verifies my loan amount. I'm sending that along with a copy of my master promissory note that I sent in as verification of that part of my funding.
I hope this helps some. I know we have a couple of people who will know definitively, so hopefully we'll hear from them as well.