1) They're fully rigged by hand, the base is handled with automatic weights and then retouched by hand, and clothes are mostly weight painted by hand to avoid weird deforms. They are also animated fully by hand.
Tool generated armatures and animations and such generally seem lacking to my purposes, as I have about a dozen or so auxiliary bones for hair, clothing and other deforms as they're not being handled by simulations.
2) Yes. The facial UV is done so that I can reuse bits of the face and morph it into the UV. Then followed by retouching to make it nicely blended.
3) Power of two. Small bits get smaller texture, more detailed bits get larger textures. You inherently know what size to use by how it looks on the model.
4) I first made the horse model, gave it good topology, rigged and finished the armature. I then turned that horse into a cow, fixed the face, hooves etc. Just by moving around the body, adding extra loops or collapsing them. Then i made it into a pig, then into a million other things. If I were to do a centaur I'd just take the horse model and pop a human on it.
Animtions are also cross-mesh and armature compatible that way.
5) Make the model perfect on first pass. A little bit of inflation deflation or verticle rearranging won't hurt the texture one bit, especially if the edit is within the bounds of the unwrap.
6)I don't, it would be too much work. Just have a single body texture that's separate from the rest and mix and match the various hairs, accessories and clothes as need be.
7) No, not required at all, if I'm not mistaken boolean merge is also a bit ugly in certain instances with the creation of n-gons you then need to clean up... I just delete the invisible body parts!
8) I'm doing no merging. Merging all textures together poses its own set of issues like the inability of ordering rendering priority when it comes to transparency and lack of flexibility. Draw calls become an issue in the high ends you'll likely not reach for such a project. At most there's only one single person in any given house and maybe 6 in an inn or guild.
9) it is a must if you want to animate the mesh. The most complex rig on my models is around the butt area due to all the bending it's required to do while sitting and contorting like they're a gymnast. Edge loops are not just for faces.
As for texture faces, that's something I won't be able to help you with and you'll need to check out animating textures for unity.
10) In a way yes. Just for parts that require such. Like eyes faces or bodies and hair. Clothes on the other hand each have their own unique unwrap.
11) I don't copy-paste the base mesh, I just make the clothes fit. This works for women as their anatomical diversity in Daggerfall is less than what we see for men. There is only extremely voluptuous and slim. Which I can easily just do by making a second body mesh and inflating/deflating it slightly, same with the clothes that fit on that anatomy. Only requiring inflation/deflation to have a fit.
12) No. I just make the model with the correct topology from the start. Sculpting and similar is not required or needed for the style I'm chasing.
This part of the project is virtually complete, All I'm working on right now is assembling it in unity.