I'm gonna go ahead and say no, I don't think Buffy was a bottled blonde. I think Sarah was a bottled blonde, because Buffy was supposed to be blonde, not because she was supposed to be a 'fake' blonde... I.e. I think Buffy's bottled blonde look was an unfortunate accident due to the casting of a brunette actress
The casting of the child actors I would guess boils down to what the casting person thought looked most believable to have been SMG as a child, and whether or not said child had the appropriate acting skills, thereby overlooking e.g. haircolor (ones sees it all the times with characters who in flashbacks suddenly are blue eyed as kids and browned eyed as adults. Can it be annoying? Sure, I mean why not spring for a blonde kid? But hey, end of the day, maybe it just wasn't a viable option.)
and that's my two cents
🙂