Hi kingandy2nd ,the wall slopes and is not a full 20 x 10 if you get my drift lol i just used the highest point and widest width as a rough calculation as it was different sizes in certain places so did not need as much as i thought .I did use about 1 1/2 bags and not one bag as i mentioned earlier lol so i edited it .Well done on your first attempt zapper.
Not sure how you got one bag to do 2 coats of skim on a 6 x 3 m area... must have been an extremely thin coat! Each coat needs to be a couple of mm think minimum.
Couldn't comment on the best way to recticfy any imperfections without seeing it, but i would be inclined to wait for it to dry and then take stock. If it needs to be done again, do it again. You've come this far so do do it properly. Just seal the wall with pva before doing it again if needed.
Yes, you should have made some extra plaster up rather than worked the tough stuff - even better if you can make a friend or loved one do that bit for you while you're trowelling
I have the opposite wall to do (well all the stair case lol ) i might buy a stop bead and do it in 2 sections on the other wall and see if it looks better as i seem to be fighting against the clock and it going off with me being a novice.If it looks good i might redo the other wall using the stop bead .I know most prefer not to do this method but at least i can have more time and then concentrate on where the join will be .