Alright, I'm the one with the crazy requests already.
We're working on some pretty complex animations with a lot of turning around and consequent texture swap, quite like what's been said here Rotate Character
Our character have a lot of unique keyposes with most images swapped and I worked out a trick to rig them right (or so I thought?). I go like this:
Import the new textures in the pose I need but instead of matching each piece's position to the idle pose in setup, I place the skeleton in the pose I need directly in setup. I then parent textures and bind meshes as I need. All animations are broken of course since I change the setup pose. To get my setup pose back, I import the skeleton from the same project (a previous version, or I import it before starting), so I can copy the position of all bones from the imported skeleton in setup and paste it back into the first.
This way when I recreate the new pose I have all pieces in the correct place without weird trial and error. I did this a few times and it results in clean rigs for the new poses and it's relatively quick
Sometimes I noticed a mesh or two turned out modified in the setup after this process but no big deal, I can get them back from the imported duplicate skeleton. Now I start to notice more twisted meshes, even in scenes not affected by the pose I just set up? I wonder what step in this rigging method does this. Maybe they get twisted when the setup is temporarily edited and all animations are affected? Any idea how I can prevent this?
I have a lot of unique poses to insert in the rig and I'd go crazy trying to align every single piece for every pose, and I can't place wheights correctly by guessing when poses change a lot from the setup.
Again, I can't divulge files, but maybe I can cut out a partial file and share it privately. @Erikari knows what we want to achieve since she's been rigging other characters though not with my setup-editing-method

At some point I tought of rigging the new pose in the duplicate skeleton and THEN move the attachments to the main one, but I'd need to parent them twice I suppose

The draw order issue (problem with draw order) happens with other character too, though. It might be related to the shader.

This mesh breaking keeps happening, especially when I do this setup trick to rig poses, but it probably appeared in other occasions in the past and I didn't notice immediately. I'd like to at least know what action causes it so I can fix them as I go and not find unpleasant surprises in finished animations
We'll try to understand at what point of my workflow this happens now

It must be related to the moment I change the setup to rig additional pieces. All animations are screwed for a while and that could influence meshes?
I said sometimes I found crooked meshes even before doing this, maybe I was editing parts of the setup and it happened

The one piece that gets crooked the most is not bound to bones with any constraint.
And by one piece I mean all versions of that same piece actually. I'm not using a lot of meshes in this character at this point, the bust of the character is the most complex and it is not complex at all. It binds to 4 bones usually. I have multiple busts for multiple poses and it breaks often
Now I'm at the point that some textures come out crooked even if I replace them with a previous version that's correct

I'm not allowed to post anything publicly, maybe I can show you something in private?
Editing in the setup is much much easier for rigging alternative poses than placing the textures one by one and trying to align them. I have a lot of poses to create and this could really speed me up if it didn't break previous works

edit: nope, can't show anything. I can explain what I'm doing in more detail if that could help

Probably, yes.
I'll describe what I do and what I want to achieve one more time for clarity:

I have a human character, in the setup mode it's posed in an 'idle' position. The character needs to move around a lot and to avoid a puppet look and have some keyposes look more like proper illustrations, I'd like to bind new pieces precisely to the bones and mesh and weigh some to be able to move the new pieces around a bit and not just swap to a still pose.
My art director in an early test used to scale a bone at a time in setup mode to bind textures. For example: if he needed a foreshortened arm, he'd scale the bone in setup, bind a shorter arm texture, then put the bone back to its basic position. This way when he'd need the forshortened arm, he'd scale the bone in animate mode and swap to the shorter texture.
After rigging some poses I thought I could bring this to the extreme and worked out a way to backup the position for all bones in setup, which is so:
I import the same project so I have a duplicate of the skeleton - and therefore a duplicate of the setup position.
Import the textures for the new position. I have exported them in place from photoshop and relative json.
In setup, move the skeleton to match the pose, even flipping and shortening bones if needed. Bind textures, weigh meshes etc as I would normally do. Now the new pose is ready and rigged. I usually backup this pose as well either by pasting the bone position in an animation in skeleton #2 or by importing another copy of the skeleton in the file (this way the new pose stays in setup)
At this point I copy the position of the bones of skeleton#2 from setup and paste them back to my main skeleton in setup. All is back to normal.
I recreate my new pose by pasting it from the backup, turn on the correct textures and I have it perfectly rigged with working weights and all

What it does that's unwanted is: sometimes I go back to setup or some animations and find some meshes are deformed. Most of the times if I still have my skeleton #2 in the file, I drag the mesh from there and replace it and all is fine

Before this I would import the new textures and align them to the skeleton in setup while the skeleton stays in its normal idle position. When I try to create the new pose in animate I need to move around the bones a lot and go back to setup to edit meshes and it's a lot of trial and error. Erika showed me how to use the preview to adjust texture but it's still a tricky process and you can't really weigh meshes in detail

Update: I tried rigging a pose without entering animate mode in the process. It looks like no mesh was deformed. Could it make sense or is it a coincidence?