Actually Akasha, I believe those questions have actual answers, with some pretty concrete support.
First of al though, who are "
they" ?
As far as I know the transference of slayer power has always been described as an automatic process based on rules created by the magics that created the slayer line.
The First slayer was created by the Shadowmen, thousands of years ago. We know this from Joss's Comic book mini series Fray (Ep. 3).
The rules governing the slayer line also appear to have been created by the Shadowmen. It's suggested pretty clearly in the following bit from Chosen
BUFFY (cont'd)
What if you could have that power?
Now. All of you. In every
generation one Slayer is born because
a bunch of guys that died thousands
of years ago made up that rule.
At no time has it ever been suggested any person or group of people is actively participating in the transference process.
In fact in Showtime Beljoxa's eye tells us there is a corruption in the laws governing the slayer line. To me this suggests the passing on is done remotely by mystical power governed by set rules, not by actual individuals.
Were Kendra and Faith in training while Buffy was the slayer? I'm going to say definitely yes for Kendra, and possibly yes for Faith.
Kendra arrives fully trained following the season of Buffy's first death, and I believe you're correct when you say Kendra said something about her watcher taking her from her family at an early age.
Faith spoke of having a watcher before coming to Sunnydale, but there is no timeframe for it.