To echo the sound comments here, this mountain in the winter could be just as tough as Rainier, without the crevasse/serac risk. So, be prepared for full on winter conditions and plan on at least 3-4 days on the mountain and watch the weather. You could also be hiking in virually no snow, so it will all depend on how much white stuff is on the ground.
You'd be lucky to get within a day's hike up iceberg lake since the road might be closed miles before the Portal.
Depending on where the road is closed you might only make it to the trailhead on day one. Getting water could also
be tough if there is a lot of new snow. You'll likely be melting snow.
As far as routefinding, I've been up the main trail in trackless, new snow in April/May and the trail was impossible to see in some places and I've climbed Whitney three times already. If you've never been up there it might be a tough day. Granted, I had no GPS, so perhaps that would help you assuming it had reception and batteries were still good. Don't rely on a smartphone GPS.
On the MR route, I assume in fairly powdery unconsolidated snow, the upper route would be challenging and better left
to early Spring/late winter conditions when the snow consolidates and hardens up to climb on.
As far as returning down the main route, could be fine if there isn't much snow. If there is, it might be easier to descend the chute to the west of the switchbacks, but be cognizant of avy risk depending on the conditions.
Look at the link below for my climb this past Spring which shows some shots of the chute - not a place to be if all iced up or if new snow has fallen creating
ripe avalanche terrain:
http://www.14ers.com/php14ers/tripreport.php?trip=11767&parmuser=nyker&cpgm=tripuserGood luck!
