"Ian or Ed- is there a preferred map(s) for the Wonderland?"
I have both the Green Trails and the Nat Geo maps. Both are fine.
Only issue I've seen with the Green Trails map is, as a courtesy, they note the mileage between points. There are a couple sections where they are off. Way off. The scale is fine so if you take the time to measure it on your own, you'll be fine.
Nat Geo Map is what most of the rangers carry (at least the ones I met). The only place where I got screwed up, and it shouldn't have been difficult, is crossing the river in a counter-clockwise direction at Box Canyon. I hit an intersection that didn't have a sign. Turned out all I had to do was walk along the road across the bridge and the Wonderland Trail resumed on the other side.
Anywho, I worked through it by using the Gaia app on my iPhone. I downloaded the entire MRNP prior to the trip which allowed for me to zoom in on a USGS map in much better resolution than the Nat Geo map provided.
"Hiking the Wonderland" has good information about the mileage and elevation gained/lost between points.
The rangers can give you a copy of the elevation profile of the trail. It's a good reference for anticipating your ups and downs of the day but some of the climbs appear worse on paper than they are in reality. For example, hiking Cowlitz Divide in a counter clockwise direction appeared to be a nightmare on paper but was a slow, long, steady climb in reality. No big deal. I'll upload a copy of it as a .jpg when I return mid September.
"Ian-do you tie the corners of the ground sheet to the tarp to lift the corners?"
Yes. A BPL member Randy (Cain?) posted that somewhere and it's an excellent technique. I've used the thermal sheet as a ground sheet and it worked fine although the rain we received on that trip was minimal so I can't claim long-term experience with it. I do have a zpacks twin bathtub floor so I may just bring that instead as I can roll up in it if need be. I have a few days to make the decision.