Stockholm City Hall tours

I think the 10:00 tour would be cutting it too close, as I read that the ferry doesn't hold a seat for your reserved-in-advance ticket if you aren't lined up at least 15 minutes before the sailing.

Looking at Strömma's website, they mentioned that you should be there 15-20 minutes before departure, not that you must. But the 10:00 tour would be cutting it close. The boats to Drottningholm depart very close to the city hall, but it will still take a few minutes to get there and it's not a nice trip if you have to run between sights.

If I understand how the City Hall tours work, you can only buy timed
tickets in person on the day of your tour, with the ticket office
opening at 8:30.

As far as I know, that is correct.

If that's correct, how likely are we to be able to get the 9:30 tour
for our traveling party of 3 if we arrive at the ticket office at
8:30? Do people line up at the ticket office before 8:30?

I have no idea. But to make a semi-educated guess I don't think you need to worry about not getting three tickets for the 9:30 tour. In the summer there are 14 daily tours in English and I don't believe the 9:30 is the most popular one.

We were in Stockholm about 3 weeks ago, and took a City Hall tour. We arrived around 9:00am, and found the 9:30am English tour sold out, probably due to tour groups. That being said, there was not a big line for tickets. I suggest getting there as early as possible. If the ticket office opens at 8:30 and you arrive shortly before then, you will probably be okay for a 9:30am tour. But if you MUST have the 9:30 tour, then get there as early as possible since you don't have flexibility. Good luck.