Congratulations on your new granddaughter! She is a beauty!
I hope you and your daughter find some answers soon, but with all honesty it might take a while.
My 2 year old son was born with a red birthmark affecting his right hip/leg all the way down to his toes. His is reticulated, so it's not a solid birthmark, some "good" skin shows through. When he was younger his also almost disappeared when he was warm, but turned dark red when he was cold or was crying.
For now his diagnosis is a port wine stain, however his doctor is keeping an open mind that it might be CMTC. His PWS does seem to be darkening, even when he's warm it doesn't disappear like it used to.
For now, we have decided against laser surgery. I keep a close eye on it, and we'll see the doctor annually unless something changes.
Also, you may want to check with your local hospitals, especially if you are near a larger city. The 2 major hospitals in our area have "Vascular Anomolies Teams." I really like and trust our pediatrician, but she didn't even know that these types of doctors were available in our area!!
I wish you and your family the best of luck! Keep us updated!!!