Too many things, below are random comments, you have googled enough, take your pick of what suits you.
- Do not get too hung up on a flat matt, you would not really tell the difference with a vinyl matt. It is the acrylic paints that start having a noticeable sheen (from my limited experience). Flat matt is fine for a period property yadda yadda yadda.
- your contract matt (i.e. non-vinyl) remains more absorbent than a vinyl paint. So if you put 3 coats on top of the mistcoat the 4th would absorb fast and would also activate the previous paint. Too much rollering and you got 4 coats to create orange peel with.
- as above if you do not let the previous coat cure and soon you are putting the second coat. Paint another ceiling and so on and let it dry as much as possible.
- the reason I asked you which direction you were painting is because the "shading" is consistent with left-right movement (but so would be ripples in the plastering).
- if you use a scuttle consider getting a tray instead (toolstation used to have some 15" or 12" wide trays for a fiver or so, wide enough to take the frame of the roller sleeve). I have only used a scuttle once (friend's warehouse) and I did not manage in that short spell to become able to load the sleeve evenly and not to overload it on one side. For what I was doing it did not matter but if you end up with a badly loaded sleeve you end up rollering more and thus more orange peel.
- Are you sure the ceiling is 20 square feet? A room 2m x 3m would be give or take 70sq feet. Not that it matters, but if your room is e.g. 2mx3m (7x10 feet) work from the narrow end towards the far end. Divide the area in blocks so for the 7ft width you might have 2 blocks 3.5 feet wide by 3-4 feet long. Paint each block overlapping as necessary ... then move forward and repeat starting from the same side with another 2 blocks overlapping those behind you. The idea is your wet edge will have to be only 7 feet wide. I am sure your room is bigger than that but try to get the idea.
- if you decide to sand (you should as Puma said so
) this is your cheap option
https://www.toolstation.com/shop/p55929 + a roll of
https://www.toolstation.com/shop/p77944 at 120 grit (no lower number, and start gently with the 120 in case you scrape the plaster). Alternatively a 1/3 sheet sander like so
https://www.screwfix.com/p/makita-bo371 ... 240v/6330j. It needs specialised sheets with holes and whatever else for the vacuum to work and you can improvise and connect a vacuum cleaner to it if you really want dust free sanding.
- do not roll too much, just enough to produce an even coat with no tramlines left from the edges of the sleeve.
- do not press the roller trying to get all the paint out of it, reload it instead and keep painting, the whole thing is a delicate process (says I that broke a 9" handle when I used it with a pole ... and it was a crap idea to even try using a 9" handle on a pole, unusable).
Maybe practice your skills in another room using normal trade paints (in alphabetic order Crown, Dulux, Johnsons or their cheaper versions MacPhersons, Armstead and Leyland) before you return to this room with the expensive Benjamin Moore stuff.
), er I mean, the towel in!
I have only thrown the kitchen sink at him 