Defrosting approx 3.5kg of pork, and then cooking more than once? I've got about 3.5kg (7.5lbs) of rolled pork (with lots of skin crackling) vacuum-packed in my freezer which has been there for just under a month. I've got around 20 coming to eat on Saturday for a buffet and I'd like to serve it as pulled pork, with separate crackling.

So, first question: when is it likely to need to come out of the freezer? 24 hours before cooking? Or 48, to be safe? It's pretty big.

And, second question: I'd normally do pulled pork for about 5 hours, but this is a larger joint than I've cooked before, and I'm wondering if I can cook it for perhaps 5 hours on the Friday, remove the crackling, then leave it in the fridge overnight, and put it BACK in the oven for another couple of hours on the Saturday. Is that hygeinic / healthy / possible? It'll do the crackling wonders but I'm thinking more about the meat.

Any other tips / advice appreciated, though bear in mind that unfortunately I can't leave it in the oven overnight since it's a temperamental old oven and I need to keep an eye on it :(
Your proposed method is possibly dangerous, you'd be warming it enough for botulism to thrive, but not enough to kill it. When you cook it, cook it properly and all at once. What I'd suggest is simply cooking it on Friday, then re-heating it saturday, or getting up early enough to cook it through on Saturday. As for when to thaw it if it's a truly heroic size I'd thaw it in the fridge starting 48 hours before you want to cook it. 

As for how long I'd usually do 20 minutes per pound plus 20 minutes as long as the meat is room temperature to start with, so for 7.5 lbs, so that's near enough 3 hours as makes little difference. Not that long really, if you wanted to serve at 1pm you'd want to get it out of the fridge as early in the day as you can, get the oven on at 8:40 on 350F (375 F on a conventional oven), get the meat in at 9, roast till 12, then rest it for an hour. Crank the oven up to max for the last 20 minutes to get the crackling extra crispy.