Hehe
Sync new posts since, etc ... reminds me of the 'cut off' date of Domino servers (back in the days when I worked with the Domino databases). If I remember correctly, it was related to when the replication last happened, so you could only replicate was was newer from the cut off date.
I am sorry but it would require too much time on my part ... unfortunately in MySql also - I don't think you have support for real-time replication, last modified date, cut off dates, etc, to help you, everything you have to implement either by hand, or ... you have to apply only the diff (between the two SQLs);
But then ... you might have conflicts - what if a board was renamed in 'production' server ? If you don't have the date of last modification - and you don't have it without modifying SMF, you have to handle each conflict manually ... do you keep the local version, the remote version, what do you keep from local ? If you keep only what is identical with 'production', then why not removing everything then reimporting ?
So as you say
it's not a complex situation to understand what is needed, but that would be too much ...
I think, at least for themes, it might be easier to modify SMF not to show some themes to the users, so you could just do an 'update' in 'production' server and see them as admin - without fearing that users might select the theme.
One thing I did was a perl script that takes a 'production' databse - replaces all URLs with the localhost address of the forum (so then I don't spend time to reset all theme URL and paths) and imports it into Mysql. It took some of the pain away ..