If I understood your question correctly, then you have the problem that you can successfully replace a region attachment, but replacement fails (with strange vertex placement) when replacing a mesh attachment.
This is due to the very limited nature of the old legacy example component SpriteAttacher, which is limited to attachments of type RegionAttachment, it will replace an existing MeshAttachment with an un-deformed RegionAttachment which will animate and place vertices incorrectly.
Please instead check out the example scenes Spine Examples/Other Examples/Mix and Match Skins, Mix and Match and Mix and Match Equip.
You may also want to check out the spine-unity documentation section Combining Skins below:
spine-unity Runtime Documentation: Combining Skins