It's been a while since I wrote the interface, but when reading your post it gave me flashbacks on using the DRDY pin.
I use timer2 as clock source (4MHz in my case) and bit bang SPI. First step is doing a reset in accordance with the datasheet. DRDY should then output a valid signal (you may want to check DRDY with a scope or otherwise to see if the low/high frequency reflects the default configuration). SPI will then be configured for MSB (most significant bit first) and you should be able to get some consistant readings as an initial test (write adr/cmd byte followed by read).
Also note that when in differential mode, the returned value will be signed (manual sign extension to 32-bits is needed however to conform to a long). My implementation is polled and I use direct port I/O to check DRDY (wait for high then low before proceeding). Also for the SPI part I use direct port io as it is "impossible" to get timing right with digitalRead and digitalWrite.