Sorry to chime in late here...

I think your questions have pretty much been answered and with good information. You will have one problem if you plan to run 15mm adapter-style spacers (if you can even find someone to make that dimension). The problem is the spacer isn't thick enough to bury the protruding lug from the hub nor the nut that attaches the spacer to the hub. This isn't a problem if the wheels you are running have large enough voids between the lug holes. This will allow the wheel hub face to sit properly on the spacer face. 20mm spacers will have this same problem.

Another issue you may have is 15mm may not be enough depth to properly bury the head of the lug in the spacer. I had a set built for a customer and we used metric lugs because the head was quite a bit thinner and the spline on the lug was relatively short. I doubt very much we could've used standard 1/2" lugs.

It's not such a bad thing to run sandwich-style spacers, provided you have the proper length of lug. When I did my car I swapped to 1800 front hubs and welded in longer studs. My spacers are actually incorporated into the rotor hat for the Wilwood brakes. There are differing opinions on the strength of adapter-style spacers as compared to sandwich-style spacers with longer studs. My opinion is either will work for 99% of the people that use them. The other 1% likely know enough about what they're doing that it really doesn't matter...
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Dale