Morgana wrote

1.- Can a duplicated placeholder be erased from just 1 of the skins?

Why do you want to erase the placeholders? At runtime they do no harm when no attachment is set.

Morgana wrote

2.- How can I duplicate an already existing placeholder to other skins?

I'm afraid I do not understand this question. A skin placeholder is not bound to any skin (most likely I misunderstood your question).
Did you have a look at the official documentation pages about Skin Placeholders?

Morgana wrote

3.- I have IK's on my character's arms and have an animation where this character is moving both arms from left to right, for that movement I've checked and unchecked the positive option to the constraints, but in some positions, his arms have like "involuntary" movements and I thought with the mixing option could fix this, but it still...

Could you please share a short video or animated GIF showing your problem?

Morgana wrote

4.- How can 1 of several skins have a different movement in animation mode?

While you could achieve that via separate Skin Bones, I'm not sure this is the proper solution for your scenario. If you don't want a separate set of bones, it is most likely the best way to simply create a separate animation.

Morgana wrote

Hi Herald, thanks for answering and sorry if I'm not clear, I'm not native English

No worries your English is fine, basically I just didn't understand why you would want to copy a skin placeholder.
As I'm a mere programmer, I'll let Erika chime in for the workflow and constraints related questions.

Hello Morgana!

I'll answer your questions too

1. emptying the skin placeholders is the correct way to have certain parts only present in some skins but no others, so well done!

2. I'm not sure I understand the final aim either: if you duplicate the skin placeholders, you are going to have copies of what you already have. I guess you likely want to duplicate the assets inside the skin placeholders, in which case: do you need to have the same assets in different skins? if that is the case, just duplicate the attachments, take them out of the skin placeholder, change skin to the skin where you want them to be copied over, and drag each part under its correct skin placeholder to make them only active when the chosen skin is active.

3. IK mix (that is to say, the amount of inluence the IK has on the constrained bones) can be turned off during animations:
   Inverse Kinematics - Spine User Guide: Mixing FK/IK

This is a perfect use case for it: key the mix as set to 100 in the beginning. a few frames later, key the mix of each IK set to 0, you'll notice the arms will likely change position back to the setup pose while you do this. This is normal.
Take them from that position to the one they should have in that position. Add a keyframe where the mix is set 0 at the end of this section to preserve this portion of animation, if necessary.
Next, animate the arms rotating to the other position. once it is reached, key the mix back to 100, and invert the bending of the IK.

4. perhaps if you can make a concrete example I may be able to help you figure out if this is viable or not using skin constraints, but as Harals has suggested, making a new animation is often the easiest solution.