Drywall – Hanging Drywall Against Finished Ceiling

It doesn't make sense to do the inside before the roof has been replaced. The drywall will most likely end up getting damaged when the roof is being demoed and new sheathing is installed. The contractors may inadvertently step on the backside of the drywall and cave it in. There is also a big chance of some water damage. Keeping the original lathe for the roof replacement would also help protect your belongings on the inside. You definitely wouldn't want to be able to see the sky by looking straight up from the bedroom.

Once the roof is complete, then you can demo the inside. Since there isn't much space in between the roof and the ceiling, your best bet would probably be to have spray foam insulation installed. A closed cell foam will be a good choice here because it acts as a vapor barrier and will help prevent moisture from damaging your ceiling. Depending on your climate, the spray foam may not provide enough of an R factor. If that is the case, you can then install XPS foam panels on your entire ceiling over your rafters, and then drywall over that. That should be enough to keep your house warm.

1/2" of dip in a relatively small space can be sort of hidden with more mud, but I'd say it's always going to be noticeable.

2 alternatives. First, get some serious leverage underneath (something wide at the ceiling and an angled 2x4 from the floor) and see if you can push the offending drywall up. It may have just been poorly nailed in the past and is sagging.

If that doesn't do the trick, I'd even consider cutting back the offending piece and re-doing it. I get that almost nobody likes drywall work, but since you're doing some anyway, the incremental agony of a bit more will be modest.