We've spent several NYE in MK.
First, if this year follows previous years, MK will open at 7:00 am for EMH and then 8:00 for everyone. I would not want to waste the best park touring time at CP. If you're there that early, enjoy the relatively low crowds for a few hours.
Second, if you're at the Dolphin, take the bus. They often close the parking lots by late morning, but we have found Disney transportation often gets you in even when the parking lots close MK off to offsite guests. Getting out at night will be a nightmare no matter what, so prepare for that or plan to stick around for an hour or two after midnight till the madness calms down.
Third, if you're hoping to see the fireworks at midnight, there's no way I would recommend getting there and staying all day. We have always stayed on site and arrived late afternoon or early evening and it's much more relaxing than fighting crowds all day.