I haven't made that trek but assuming you use the side entrances and not the fronts to each should be able to progress back somehow. I would think I a cab might be a real bitch but hard to tell as they might be able to take Dean Martin down toward Mandalay from the park area.
Was able to move between aria and Cosmo no problem on the bAck entrances 2 years ago.
They close the strip around 6:00, so you wouldn't be able to cab it, I would walk over to Excalibur and then take that tram to Mandalay Bay. We have done NYE a lot in Vegas and have never been asked for a room key to get in a hotel, I suspect those that do check it would be very close to midnight. I'm definitely up for a meet on the 30th!
Might be a whole different ball game this year with being able to walk between hotels and security this year compared to the past.