Hello
I didn't made tests with Gui Scripting.
I put my hands upon two pages from
Learn AppleScript
The Comprehensive Guide to Scripting and Automation on Mac OS X, Third Edition
by Hamish Sanderson and Hanaan Rosenthal.
The problem isn't a new one and it's even worse than what it was when the book was written in year 2010.
At this time your original syntax : make new mailbox with properties {name:tempYearString & "/" & TempMonthString}
worked !
Sounds like a bug report was required. As you are more fluent than me with English, I let you file one. If you really did, you may pass me the given report number so that I may send a report just telling that I'm also facing the same oddity.
Maybe passing the two attached pages in a report would be a good idea. It would add a voice and would highlight the fact that the situation is worse than what it was in 2010.