You have a bunch of questions here, so I'll try to hit up a few major points. First, I'd suggest joining the AMC. It's just $50 and it will get you some discounts on the shuttles and at the huts (should you choose to stay at one for a night). Also, you should pick up a copy of the book, Not Without Peril, before you go. Seriously, it will give you a great appreciation of the history of the area and it's just a really interesting read.
"7 Days at ~20 miles/day. I'd like options to shortcut out if I can't keep pace as well as options to tack on loops if I get well ahead of pace. Getting to my car a day or two early and then driving somewhere for an aditional 1-2 day loop is fine."
There are plenty of options in the Whites. You should pick up a copy of the latest edition of the AMC trail guide. Some of the older editions are too "poetic." I would STRONGLY caution you against planning to do 20 mile days in the Whites. The weather is too unpredictable and the terrain is generally too rugged. I've done 25-30 mile downs in the Green Mountains of Vermont, but unless you're an extreme outlier and extremely lucky with weather, 20 miles would be really hard to maintain. Look at some trail journals to get a better idea of mileage.
"I'm looking for either a loop or a point to point provided that there is a reliable shuttle service (cost is not a big deal). I won't have two cars and don't want to hitch hike."
There is a shuttle service. Here's the official link with a map and rates.
http://www.outdoors.org/lodging/lodging-shuttle.cfm
"PLEASE GIVE ME SAFETY TIPS. I understand weather can be dangerous there."
One of the best resources for trips in the Whites is the Mt Washington Observatory website. I always take a look before I hit the trail. Even still, the weather can turn on you in a flash. Last summer I was on top of Mt Madison in Class 1 hurricane strength winds. At one point, I was blown a good three feet to my left, even though I had my feet planted and trekking poles. This was on what had been a beautiful summer day an hour or so before.
http://www.mountwashington.org/weather/
I hope some of this advice and these links help! You may just run into me on the trail this summer.