ECG_Kinsey I don't think the IK constraint has anything to do with this case in particular, the only issue is when you set the parent of the image attachment to a corresponding bone. The following is a very simple example, but I am assuming you are looking for a rig like this, is my understanding correct?:

Here is the Spine project files:

What I did should be almost identical to what you were trying to do. I imported images of another character and adjusted the transform of the bones to match that character's arms using transform constraints, then added the constraints to the character's skin. I then set the parent of the images to the corresponding bones. This allows you to place another character's attachments without affecting the position of the first character's attachments.
I hope this will be of help to you.

ECG_Kinsey (Except for the IK issue you just posted, this is an answer to your earlier question on the matter.)
I think your understanding that you need to move the attachment is correct, but it's hard to explain well in words, so I've recorded the process of setting up the example I just posted above and uploaded it below so you can take a look at it:

ECG_Kinsey Hmmm, as far as I have tried on my end, changing the bone scale in the transform constraint does not produce that result. I've attached a updated Spine project with the results of my own testing (in skin B I scale the arm bones via the transform constraint UpperArm_Position_B):

Perhaps the quickest way to confirm your problem is to see your Spine project in person. A minimal Spine project with irrelevant parts removed would be fine, so could you email it to us?: contact@esotericsoftware.com

Bone and attachment (Images) compensation affects manipulating bones in the viewport. It does not affect applying constraints. It is not applied when adjusting a transform constraint.

You are fine up to this step:

If I understand, you parent the slot to the root bone, apply the transform constraint, then parent the slot back. That puts the arm in the right position, but in your original skin the arm moves. Is the arm that moves in the same slot? If so, then you can't use parenting the slot to the root to move your attachment. Instead, parent only the attachments in the second skin to the root bone in a new slot, apply the transform constraint, then parent them back and delete the temporary slots on the root bone.

As a simpler test, instead of changing the parent of attachments in the tree, just manually move the attachments to be in the right place in the viewport. Each attachment has its own position. Moving one doesn't affect other attachments in the slot or in other skins. (Except for linked meshes, which share the vertex positions.)

For your IK scaling problem, you probably need to set the transform constraint to local and possibly relative. In general it's not possible to tell what is going on from an image or GIF. If you can send a .spine file it would be easier to provide guidance.

Sure, we're glad to help!

It is a little unfortunate to need to disable inherit scale to change bone lengths for skins, and to have to introduce intermediary bones for attachments. We'll consider other ways to change the bone length. For it to be in the skin, it'd need to be a constraint.