You can compare the FDR data with the timeline of events and get a shred of information about cabin altitude. Passenger Oxygen Caution came on about 7 minutes after the downhill ride began, well after the conversations you've quoted from the CVR occurred. Cabin Altitude Warning was on immediately following the FDR gap, but not prior. (The gap was due to loss of electrical power after the engines flamed out. Data collection began again once the APU Generator was on.)
While the cabin altitude would have been higher than they'd experienced previously, the poor decsion-making didn't begin there. Poor decision making is evident just after rotation, and it had probably occurred prior to engine start.