20 months is not fixed for everyone; it's statistical calculation and it's an average for 80% of the cases; let's say if 100 r done in 16 months and 50 have some issues and r done in 23 the average time will drastically increase; so u could b in the lucky 20% and b done much sooner or in the not so lucky other 20% and b done even later than 20 months; but if ur file is finished they won't just hold it there just cuz it's below 20 months - i think it's in their interest (work productivity, bonuses, etc) to improve their average; however they will use that time to make u not bother them, inquiry abt ur case so on
as for the 2nd medical welcome in the group, many of us have to do it or already did it by themselves or have the embassy asked for it and soon after got their visa; i think ayad's wife also was asked for the 2nd medical, not sure how long ago - maybe 1 month?! if u already saw it on ur ecas i think the final decision is weeks close but since i'm not in that situation i cannot say exactly how long it can still b; even if others got it soon after every case is different so all u can do is hope
good luck!! finally we'll have some good news from u soon