The reason I said not to knock yourself out on the work needing to be done is exactly what happened. The recommendation you seek on leaving was going to be mediocre/lukewarm and so will any recommendation letter. It will be complimentary but won't talk about your great work ethic and effort you've put in over your time with the company. It's because you gave well-meaning advice that they didn't want to hear.

It just seemed to me like that was how it was going to go. So again, if you want to work overtime and put forth all that effort and deplete yourself because you feel it's the right thing to do, great. But you are probably going to get a favorable, if lukewarm, recommendation whether you kill yourself doing the work or not. I'm a person who will not put energy into a sinking ship, so my personal choice would be to preserve my energy for my fresh start in a new company and just do an adequate job on the things needing to be done at the present one utilizing the time I have left (without spending extra time outside business hours).