By the most widely used convention, which has Fates as 14, it's pretty decisive that Echoes gets counted. I guess you could make a case for Heroes being counted as well but I think it's better to keep it separate, like with BS FE. Realistically, if someone says "FE15", they're probably meaning Echoes and if they say "FE16", they're probably meaning Switch, and I think it's a pretty safe bet that they'll tend to be interpreted as such.
If we were to go by current "official" numbering, from what I understand, BS FE is FE4, Radiant Dawn is FE11, Fates is FE15/16/17, I guess Heroes is FE18, and Switch is FE20? And that's just assuming the crossovers aren't counted. Those numbers aren't going to do anything except confuse everyone.