Answers to my own questions for the record:
Yes, the tour happens first and lasts about 1 hour.
Yes, the walkthrough is about 1 hour.
If there's nobody at the main gate, it probably isn't so easy to find where to go. First of all, don't go to the service center, go right from there to get to the main gate. After you turn left into the main gate, you do need to shift right a bit [divide by 2]. Several of us missed that and parked too far North.
The tour left maybe 15 minutes after the scheduled time.
They weren't counting guests at my tour. I think Tesla should make it clear via email that there's a limit to how many guests you can take. One gentleman brought 10 people with him (11 total). I asked how many cars he was picking up, and he looked at me strangely: "One". The tour guide commented on how it was the biggest tour group she had ever seen. (well, yeah, that's what happens when 5 people picking up cars results in 25 people on a tour because no email told us we were limited). I felt silly for being a party of 2.
They didn't seem picky about the age of guests. There was an infant, lots of teens, and a 10 yr old. [Amusingly, I thought the 10 yr old asked some of the more insightful questions.] The tour is all on foot and it does involve a handful of stairs. Nothing significant, but notable for some of the less mobile. The tour is very low key. We were allowed to walk where we wanted as long as we were with the tour group and not in the way.
One group arrived two hours late for their pickup, and it seemed that Tesla thought this was typical. They might not have gotten the tour, but I'm sure they got their car.
I loved the casual feel for the tour, but I have a feeling it's going to end badly some day when some 6 yr old with inattentive parents either hurts himself or somebody's car.
Anyway, I loved the tour.