Well, Witnesses have religious conventions called "assemblies" or sometimes "conventions" and these happen about 3 times a year (give or take). No, it's not like the entire audience gets baptized at once! There is a pool usually located at the front, near the podium, pretty small, like a jacuzzi size and two or three older men, usually ministerial servants are there in the pool to baptize you. Only one or two people are in the pool at one time to get baptized. It's not an impersonal experience, it's very personal. Before you're baptized, you go through about 100 questions (literally) of questions about the organization, God, family etc. all based on specific Scriptures. Then, once you're studied up, a couple elders go through the questions with you to really make sure your heart is in it. If they don't think you seem ready, then they don't let you get baptized till they think you're ready (I've never known anyone who is actually NOT allowed to get baptized, unless obvious stuff-like they were still smoking or living with their boyfriend or girlfriend or whatever).
And as for you saying that Witnesses treat the Watchtower like the voice of God, well, technically you're right....Witnesses believe the Watchtower organization is the only mouthpiece of the "true God" or "Jehovah," but it's not like we explicitly said that. I mean, as a Witness you truly believe you're doing everything for "God" and "Jehovah," NOT "the watchtower organization," which I guess in the end that's all being a Witness is really about. So, to her, she's thinking she's getting baptized for God, and the Watchtower organization itself is kind of secondary. For example, Witnesses have at one of their weekly meetings the bible reading where they literally go over the entire bible (only 3 chapters a week though) and the congregation gives their comments on what they thought of those 3 or 4 assigned chapters....So, it's not like Witnesses ALWAYS have Watchtower aids to study the Bible...anyway, that's my two cents.