@Brian – Fisrt an observation: in the UI it's the same behaviour. You cannot move a page to a spread with the move command. You just have to drag it there.
See the following example. It shows the state of the pages after you tried to move another page from a different document to spread 2 (this should be similar to your situation). Page 4 is the new page transferred to the document:
Page 4 should stay with page 3 in the same spread.
Try the following script snippet which moves page 4 after page 3 in the same spread:
var d = app.documents; d.pages.move(LocationOptions.AFTER,d.pages);
It sounds silly doing that, because page 4 is already ordered after page 3, but look at the result:
I would be interested to see what the outcome would be if you had all the pages set to shuffle. I think in that case you might actually get the behaviour I want. Also to move after a spread it seems like you should be able to set the destination reference to d.pages.parent to target the spread instead of the page as the destination.
In any case I found an easy, obvious, but initiall counterintuitive fix. All I want to do is use landscape pages. So, instead of changing that using the page tool or changing the paper size, I just rotate all my content and use regular old pages. Now I get the desired behaviour with my existing code. Seems in some ways like a workaround, but I'm happy to be able tomove on.
Thanks for your interest and attention, I appreciate it!