So, just to get this clear in my head - if DD were to get better grades than predicted, she could immediately phone up a Uni with clearing spaces (that may have been ruled out earlier in proceedings due to thinking grade requirements out of reach) and could possibly be offered a place?
Just to make sure this is clear, it's nothing to do with predicted grades. Predicted grades have no formal/official status. It's if she gets better grades than the conditions of her offer, then she can use adjustment to apply for a place on a course with higher entry requirements. There's no entitlement, though: the institution/course with the higher conditions has to have places available and be willing to take her.
Assuming a student attains the required grades and is accepted by their confirmed first choice on UCAS, how long do they have to accept or reject the place? Do they have to have rejected it before phoning any clearing unis?
They don't have to do anything. The place is confirmed automatically. If they don't want it they have to use the UCAS system to decline the offer and release themselves into Clearing.
You can phone other institutions about what places they have in Cleaing but they won't be able to do much with you until you have a Clearing number from UCAS, which you can only get if you're holding no offers. Some courses might be willing to go through selection processes (e.g. interviews, viewing portfolios) and tell you that they will offer a place if you apply, at which point you have to decline your existing offers(s) and get a Clearing number