Creationists still believe there was a canopy. It just covered a spherical Earth. They say the canopy was like a second ozone layer, which is part of the reason for the longevity of the individuals mentioned in Genesis. (The other being the atmosphere boiling you already mentioned. They claim that the concentration of oxygen was at least twice as high.
As for my opinion: you’ve got to remember that this is pre-Babel. All the humans are in one place. There’s no reason for a flood that’s designed to kill off all the humans to cover the entire planet. Only the human world need be destroyed.
If the water is falling to Earth it is exchanging potential energy for kinetic energy which is then converted into heat when it hits. This is far more heat than the Earth can radiate within the 40 days+nights bit. That author assumes that enough water is added to the surface to cover Mt Everest + 15 cubits (the depth listed in Genesis). The author also assumes that the water came from the high Troposhere (about 10 miles up). All the rest is math.
I cribbed this from the same place. 2/3rds of the atmosphere (by weight) lies below the elevation of Everest. So you would push all the atmosphere up by 5.5 miles. But looking at it I way over spoke. There would be increased atmospheric loss (there is atmospheric loss occurring right now after all). And it would be increased, but not by any significant amount. Not in the less than a year kinda time frame. So consider it withdrawn.
After 40 days… maybe but I doubt any would still have leaves or fruit. After 150 days, I really doubt it.
After the Flood, God gives Noah permission to start eating animals, presumably because the land was not yet suitable for growing crops and all the plants had been devastated:
On the other hand, the dove returned with an olive branch (which I assume had leaves on it) so… I dunno.