Jen, yes that is quite possible... although certainly not with the Havefordwest lines as they are quite distant
But
my direct paternal line is from around a little parish called St.Issells near Amroth on the Pembs Carms borders a little below the linguistic line...
So I wonder whether they came from the welsh speaking part or Carmarthenshire originally ...
I have though, read a book on the landsker line and it says parished specifically as St Issells were anglicised late in the mediaeval period compared to the most of of Sotuh pembs which was fairly early on.