For those who may be following this thread, I just heard from DeAnn Monroe Steely who was born and raised in Rapides Parish, LA and has been researching Garlingtons and others in Calcasieu Parish, LA far longer than I have. DeAnn tells me that among her own Clark ancestors, she has two instances of male ancestors (not Garlingtons) whose name was spelled "Hubbard." However, their name was pronounced "Herbert" or "Hebert."
DeAnn tells me that she has found this "Hubbard" to "Herbert" or "Hebert" translation not only in Calcasieu, but also in Evangeline Parish (formerly St. Landry Parish)
In case any readers want to contact DeAnn on this, her email is firstname.lastname@example.org.
This seems to be one more (unexpected) bit of support (in addition to the handwriting analysis described on my first message in this thread) that the "Hebert Garlington" found in the 1870 Calcasieu Parish LA was, in reality, "Hubbard Garlington."