Dearlove Decorators

That would all depend on the size of your room to how much you need but i would suggest 2.5l in order to get two full coats on the woodwork and have some left over for any touch-ups just incase it needs it in the future.

I would consider the time it takes the painter to prepare the room and paint (including drying times) which would work out at 1 day to prep walls and ceiling and paint the ceiling and 1 day to paint the walls allowing drying times between coats and the tidy up at the end. i hope this helps.

It could be the type of paint you are using, matt paint should leave you with a nice flat smooth wall but if you are using something with a sheen to it like silk it can leave a streaky roller pattern in certain lights and angles.
if this isnt the case and there is patches where the paint will not adhere to the wall i would suggest painting the whole wall in coverstain, a light sand down and recoat with desired paint with 2 full coats allow corect drying times and give it a 3rd coat if needed. i hope this helps

The woodwork should have been sanded during the prep, you are right but sometimes it does not work out like that... if they are careful sanding and clean the dust off the wall it will be fine. i hope this eases your mind.