Interesting points. I always thought that Elizabeth Montgomery made a small effort to show an influence on Sam's speech from Endora and Maurice's accents. That, or in real life she actually did say "raw-ther" for "rather" ("Mother, I'd raw-ther do it myself!"), and "cherry to-maw-toes".

Sam was a world citizen who, for the time, identified herself as American. I think that, in her power to do so, she adopted an accent to suit. And fickle Serena seemed to adopt different accents on a regular basis!