You are overthinking it. 🙂 I honestly don't think the neck is that important. For the other measurements measuring flat is fine. Same with sleeves and the bloomers (that's the proper term for these, not pants). Just be sure to clearly state whether your measurements are all the way around or measured flat and from what point to what point the measurements are.
Some other things important to doll collectors will be what type of closure the dress uses and the type of fabric. Collectors will want to know if the fabric has any give to it and could be possibly stretched that extra half inch of their doll's measurements. I inherited an antique bisque doll from my grandmother and searched for clothes to fit her for literally decades. As long as you're clear how the measurements were taken collectors can apply that to their doll.
Oh, and it's empire waist, not umpire.
