A lot of people are probably going to jump all over me for saying this, but here it is, anyway. I think that it's quite possible that Giles being Buffy's Watcher was just a case of him being at the right place at the right time. As has been pointed out, Giles most likely wasn't the most popular Watcher, and it stands to reason that being assigned to monitor the goings on around an active Hellmouth would be a sure way to get him out from underfoot. Counting vampires in Sunnydale is probably the Watchers' equivalent of counting penguins at the North Pole (yes, I know, penguins are at the South Pole. That's my point) Sending him there, under cover as a high school librarian was nothing more than gruel and unusual punishment.
When Merrick was killed, there most likely wasn't anyone else qualified to supervise a Slayer on short notice (those who were so qualified probably already had Potentials who they were training) so either the Council somehow manipulated Joyce into coming to the town where the closest thing to a "real" Watcher was available, by offering a job managing an art gallery (most likely) or just sheer dumb luck landed Buffy and her mom there.
Whichever the case, I seriously doubt that it was originally intended for Merrick to take Buffy to the Hellmouth, because, as far as we have ever seen, she's the only one who we've ever seen doing anything like that.