After the 40 days of rainfall and 150 of the waters surging, the rest of the Flood period was the time it took the waters to dry up. Rashi (to Gen. 8:3-4,13-14) gives the timeline as follows:
The Flood waters reached 15 cubits above the highest mountains (ibid. 7:20). They started to recede on the first of Sivan, at the rate of 1/4 of a cubit per day, so it took 60 days for the mountaintops to be revealed. This was on the first of Av.
Sixty days later, on the first of Tishrei, Noach saw that the water was gone, but the ground was still muddy.
After another 57 days, on the 27th of Cheshvan, the earth was completely dry.
Ramban (8:4) raises various objections to Rashi's timeline. He has the waters beginning to recede on the 17th of Nissan, then 73 days from then until the mountaintops appeared (on the first of Tammuz), 90 days from then until the water was gone (first of Tishrei), and 57 more days until the earth was dry (27th of Cheshvan).
Now my questions:
According to Rashi, how indeed would 60 days have sufficed for the water to recede all the way from the mountaintops to sea level?
Even according to Ramban, if it took 73 days for the water to recede 15 cubits, how would the rest of it have disappeared in just 90 days? (It is true that he uses the analogy of a stream, where the rate of its recession accelerates as the water level drops, but this seems all out of proportion to that.)
According to both of them, why then did it take so long - almost two months - for the ground itself to dry out?