Don't be alarmed. The decorations...the snow on Main Street...it's just very magical there around Christmas. New Year's Eve is VERY crowded, but the CMs at the Parks do a great job of helping you get around (even backstage as necessary).
I definitely wouldn't do the "there before the gates open and leave at Noon bit." I have a 7 year old (our first Christmas trip was when he was 5). We did the opposite of the norm after 2 days...we call it our Christmas Schedule. We get up late, eat a leisurely breakfast, take a swim (sometimes) and then get to the parks around Noon to 2 pm (when those with little kids are leaving). We found that we could do any ride we wanted to after 10 pm with litte or no wait (compared to 1-2 hours earlier in the day). We also use the fast pass system and go to the little known (and lesser used) attractions such as Belle's Storytime, the Carousel of Progress, etc.
As for what happens on NYE...I've only ever done NYE at the Magic Kingdom. By 10:30 am the park was closed. We had a great day though. We saw both papades (the afternoon and the SpectroMagic), saw WISHES, did a bunch of rides, got hats and noisemakers (free) ate some great meals did some shows and then did the countdown to midnight with the rest of the park and watched the fireworks at midnight (which are wonderful). After that (we were strategically located) we ran to Tomorrowland and did a bunch of rides without waiting in any lines!
If you are staying off property, I'm not sure what Disney's policy is about getting in on NYE...you might want to call and ask. I do know that the line of traffic (cars) waiting to get in while our Disney busses breezed by was LONG!
I guess what I'm saying is that I love it at Christmas and don't let the crowds bug me. We just go with the flow and don't expect to be able to ride rides without a wait. Besides, a "bad" day at Disney (if there ever is such a thing) is still better than a "good" day at work!
As for MGM...you gotta see the Osbornes Lights! They are spectacular. Yes, there are lines...and lots of people...but there is still tons of magic and the CMs are great...if you are nice to them...they are VERY nice and helpful to you.

Don't have anxiety...just have fun!