"Is having 40 rentals in a calendar year the only way to qualify for PC?"

It isn't, it can be done in 30 by using a non US/Canada address for membership

"Or can it also be earned via # of rental days?"

I don't believe so. It has been reported on here that it is possible to attain it based on expenditure, for the US, USD7000 is the figure I seem to recall being bandied around.

"YTD I am on my 5th rental and my 18th rental day. I am looking at 13 guaranteed rentals on the year at 60 days. Of course there is a huge possibility (98%) that my estimation will double, but the example would then only be 26 rentals at 120 days, which still falls way short of the 40 rentals required."

If doing around 26 rentals, I would suggest moving your account to another country and then seeing if you can get the extra 4 somehow. If you could do few rentals where rather than renting for 3 days, do a 2 day and 1 day rental , that would help climb the remaining steps

Dave