I haven’t tried it myself but I believe that the following applies for MaxPass during early entry (EE):
Availability to book during EE is dependent on the day & for select attractions only (I believe mainly Radiator Springs Racers & Guardians of the Galaxy in DCA and Space Mtn in DL).
The earliest FP window would be at official park open time.
Because of the above, I would probably spend my first 30-40 min focusing on rides because you’ll be able to walk-on a bunch, only pausing to look for your first FP if you do end up waiting in a short line.
Also, on your question re: what rides are open during EE Disneyland is Tomorrowland minus Launch Bay (& maybe probably Path of the Jedi, but I’ve never bothered to check it) & Fantasyland minus Storybook Canal Boats, Casey Jr Circus Train & I believe it’s a small world also, but the other two for certain. In DCA it’s not every ride. It’s been different both trips that we’ve had DCA EE but I believe for sure it’s all of Carsland & Guardians of the Galaxy and then it varies on what’s open around the pier. I believe usually California Screamin & TSMM.