from your list the only by the slice places are
L&B soumoni gardens
I would add Joe's in the village to the list of slice places to try.
Just to be clear, DeMarcos is closed. I was there in September and walked by, and the place was gutted. Confirmed it on sliceny.com.
Also, to be honest, and speaking as someone who lived a block away from Joe's and grew up hanging out there with my buddies, I can tell you that Joe's in its location on the 6th Avenue corner is nothing at all like what it used to be. When Giuseppe was making the pizzas, back in the old days, in the now-closed Bleecker St. location, it was heavenly stuff, and that's when Joe's "made its reputation."
Now, however, Guiseppe is long gone, and the pizza there is what I'd call average, maybe slightly above average on a good day. Absolutely nothing special. I've been home enough to know (I do wind up having at least one slice at Joe's when I go home, if for no other reason than their proximity to my old place, and the fact that they're open early and late in the day). Its continued relatively high Zagat ratings and continued recommendations from people are largely based on its inherited reputation from years ago.
If you do decide to go try them out, do yourself a favour and walk a block or two northeast and go try Ben's Pizza on MacDougal and W. 3rd Street, past the McDonalds. Once Joe's went downhill years ago, Ben's became my personal local favourite pizza, and it's still really great-tasting in my opinion. Lightyears better than Joe's IMHO.
You could also try Pizza Box, just about three blocks eastward on Bleecker St., and not just because I'm friendly with the owner there, Julio. My wife practically forced me to live there on our last trip...I've always liked their slices, but she loved them.
The only other really well-known pizzeria in the area is Bleecker St. Pizza, on 7th Avenue in the other direction...I tried it recently on one of my trips home, and didn't think all that much of it, but you might as well try it too if you're on a Greenwich Village Pizza tour
EDIT: I didn't see the bit in the OP's post about being more interested in history than quality. If that's the case, Joe's should probably be on your list, although the Joe's that closed would've been a much more historical place to visit than the current location, which has only been open about ten years. Ironically enough, the new location inhabits what used to be "Golden Pizza" pizzeria, which was my *other* favourite pizza growing up! I used to split my days and nights between Golden Pizza and Joe's on Bleecker....never could make up my mind which was better.
And even though you're interested in slices, if you really want pizza history, you owe it to yourself to go to Lombardi's, and check out the coal oven if you can. Regardless of the pizza, which cannot be ordered by the slice and also seems to have slipped a tad in the last few years, a historical tour shouldn't miss the first pizzeria in the USA!