I used on WM Summerlite bag on the AT this year, but didn't start with it. I started (NOBO) on Feb 25th, just a few days earlier than you plan to. The Summerlite definitely wouldn't have been warm enough for me, nor for most I think. A lot of folks had 15, 10 or even 0 degree bags and were happy with them. I used a 20F rated WM bag, but also carried down booties and a Montbell Alpinelight down parka to wear inside the 20F bag at night and those were enough for me down into the mid-to-upper teens on the coldest nights, but certainly not overkill.
I carried a Contrail for most of the trip, and I mean carried, not used too much, because starting that early and keeping up a decent pace meant that there was almost always room in the shelters.
Looks like you've thought through your gear pretty well; best wishes, have an excellent trip!