You cannot take all cvt oil out, you can only take half oil out. For example, my car is 2016 corolla altis 1.8 cvti, and total cvt oil capacity is 7.2L: On opening nut and removing level check tube from it, it releases 2.6L oil only. So I have to repeat the procedure twice to refresh almost 80% of oil.
Ask dealership to do the same, open drain nut only. Calculate oil in empty bottle of some oil or measuring cylinder. Fill in the exact same amount you have taken out.
For second part, when you open drain nut gearbox will be at 90 degrees, it takes at least 3-4 hours to come in between 35-45 degrees, you can check oil level at that point by playing a little part with ecu of car. Dealership won’t spend this much time on your car, that’s why I suggested you easy hack above to change oil. In that way you’ll be able to refresh your cvt oil and oil level will also remain the same.
cvt oil should be changed sharply at 55k kms or 2 years whichever comes first!