1: Score the length hard with a stanley knife then snap over a long straight edge. Or at least thats how I did it when I was working as a builders boy.

2. No idea.

For long cuts you need to do what was mentioned above, scoring it with a stanley knife. If you're cutting out details like boxes or holes then a plaster board saw can be bought.

9.5mm will be fine for a ceiling.

You have two choices, you either get it taped and jointed, which means you'll need tapered edge boards. This is basically filling in the joins of the different boards, again it must be tapered edge. That would probably be cheaper, you could then paint the whole lot.

If you get normal boards then you'll need it skimmed all over.

With both you're gonna need to mist coat the finish, basically paint mixed with water. The initial coat will be sucked into the new surface so this will stop you wasting paint and make the other coats easier to apply.

Cheers guys.