The thread title says most of it—I’m printing a 10-page booklet (2 sheets of paper), and Dorico is inserting a blank page at the beginning of the layout, which causes the booklet to be off by one page (such that page 1 appears where page 2 should, and so on), and the page numbers (while correct) are on the inside of the booklet instead of the outside. In Engrave mode, no blank page appears prior to the first page of music. I’ve tried fiddling with the duplex printing and page setup options to no avail. For what it’s worth, the preview Dorico gives in Print mode accurately reflects how the music prints, showing me an entire blank page of paper prior to the first page of music. Every other booklet I have printed (4 pages max, so one sheet of paper) prints correctly. Am I missing something?
I believe this is the intended behaviour. The rules I came up with for how we should handle booklet printing prefer two pages before a page turn if possible, so if you have 10 out of 12 pages filled, you’ll get a blank front and back page so that you get two pages of music to start with, on pages 2 and 3. At the moment there are no options to control this, but we could conceivably add some in the future. I don’t suppose as things stand you would be happy to add some kind of title page to your layout so that it becomes 11 pages long, in which case your title page would then of course become the front page of the booklet?
That reasoning makes sense enough, as your booklet rules mimic “real” bound editions of music, wherein the music for, say, a 23-page Beethoven piano sonata will begin on the left side of a two-page spread. However, the Print mode preference for two pages prior to a page turn in booklets confuses me in practice, as the printed result doesn’t match what I see in Layout mode. All the page break decisions I make in Layout mode are based on the first page of music being the first page of the booklet. In theory, I would think it would be best to have Print mode booklets match whatever Layout mode looks like?
At any rate, in practicality, I’m easy to deal with. 
The title page workaround is sufficient in this case. Alternatively, I assume I could also expand the spacing to fill 12 pages, or compress it to fill 8 pages, and have the booklet print 8 or 12 pages of music without any blank pages?
Brief update: I’ve expanded the music to fill 11 pages, and Print mode is showing me a 3-sheet booklet layout with music on the front and a blank back (12th page). This is certainly a workable solution! (Though I would still advocate for Dorico to match match the output of Print mode booklets and Layout mode.)