just got around to reading all this, SB, dunno how i missed it before.
praises first; bike's looking great! i'm all into the subtle details, and the dragonfly light and grips are a nice touch. i know i've said Worksman's were "ugly," but i guess it's all in how it's done. can't wait to see more, especially the sidecar. i'd love to build one myself, but it's just so impractical with southern california traffic.
now for some tips.
your seat post shims should work fine. i've done that myself plenty of times. the smaller diameter posts usually came with a shim on those old bikes. it was split down the middle and bent over at the top so it wouldn't fall in. i've seen them on ebay.
i made one awhile ago using a piece of aluminum pipe the same diameter of a seatpost. i had a steel pipe about 6" long and the same OD as the seat tube on a bike, and i cut the aluminum pipe about 1/4" larger, then slid it inside the steel pipe.
the 1/4" that stuck out, i used a round punch as a dowel and flared out the pipe by beating on it with a hammer so it mushroomed outward and made a lip. then removed it from the steel pipe, grinded and polished the hammered end, cut it down to 2" and slit the side.
worked perfect, wouldn't fall into the bike, and was barely noticeable. kinda a lot of work just to make a shim, but you know how it is when you start obsessing over tiny bits.
as for the chainguard tabs, i had to remove one from a buddy's bike, and lemme tell you, those welders at worksman don't mess around. i started by carefully grinding the welds at both sides, figuring, like you, it'd just fall off, but it was on there pretty good. did a lot more grinding and eventually got it off, but it took me awhile to get it all smooth. as far as his bike, though, it didn't affect the actual seat tube, just was a pain to do.
and on the sprocket, we used a 36 tooth. plenty of room, and makes the bike easier to pedal, when you have to. i actually use a 36 on all my bikes, even the skiptooth (which would be an 18tooth.) if you wanted to keep the bike "factory correct," you could get a 36t from Worksman for 10 bucks. just make sure you get the right one, 'cause worksman uses thicker chains, but they do have them in both thicknesses, i checked.
that's it from me. looks like you're off to another great bike...