Hey, Tom, I can't answer any of your questions, but I did find these threads after looking around for a while:
http://forums.civfanatics.com/forumdisplay.php?f=172 It's an active forum, with stuff from 2011, which is always a plus.
There are some files here, but the site is closed and some stuff might be unavailable and links broken:
http://civilization4.filefront.com/ (but it does have some terrain and city mods for Dune, so kudos!)
They make lots of leaderheads, but also buildings, units, etc. It takes some work to find stuff, but there are good stuff (and regional flavours, like mods for hungarian units and buildings, or oriental, or apache, or whatever).
Can you post (or send me by e-mail) a couple of those screenshots from civ5 so I can have an idea how hard it would be to merge them?
