I'm not 100% sure what the practice is on sorting subcategories within their parent category.  I just checked, and I noticed that Category:Historic districts in Massachusetts has the pipe character and then "Districts" after it, so it sorts into "Districts" when you're browsing by letter.  That means you have to go to the third or fourth page, or go to the "D" listings, to see the historic district subcategory.  That sort of makes sense, but it still doesn't get it listed on the first page.
 The reason I created the subcategory for houses is because there are over 3400 entries in Category:National Register of Historic Places in Massachusetts, which is absolutely huge.  I'm using Wikipedia:AutoWikiBrowser to recategorize the houses into their own subcategory, so that'll organize them somewhat better.  Eventually, I'm hoping to set up the infobox generator to check the historic function of the property, so if the function is listed as "Single Dwelling" (code "01A"), it'll automatically put the property into the houses category.
 Speaking of the infobox generator, I finally had time to implement two of the changes you suggested.  I fixed the bug where seconds of latitude were being listed as 60, and I added the " | coord_parameters = region:US_type:landmark" parameter.  I haven't done the last-name-first suggestion yet.  I was hoping to find a way to automatically try to reverse the name to try looking for the last name first, but I haven't fully figured this out yet.   (Elkspeak)