I hope my answer didn't seem rude, it wasn't meant to. It just seemed like you were about to put a lot of effort into something that shouldn't need to be done.
My answer may not have been the most insightful, but the fact is that I don't know exactly where Moodle stores the 'effective from' date of manual enrolments, and we don't have any 'scheduled manual enrolments' in our Moodle database so I can't even look for you. Even if you did find it, it's stored in Unix timestamp format, so it won't look like this: 2013-01-25 00:00:00, it'll look like this: 1359072000. To bring it forward 5 minutes, you'd need to change it to 1359071700.
If I had to guess, I'd guess it would be stored in one of the customint fields in the mdl_enrol table, but that's a total guess.
I was trying to explain this in a neat way, but my brain kept saying "just advise her not to do it!"