Here's how I would do it, but doubtless Mauvemorn will have advice that's better than mine. I’m working with your “Sage Hair After UV Fix attempt.blend.” I’ll try to be succinct (Lol, not possible)
Prepare the mesh: *Delete all meshes in Blender except one (which you will duplicate later for your other cut numbers after fixing the mesh).
*Tab into edit mode, press A , then M > by distance to remove doubles
*Your normals are flipped in the headband (the mesh appears sort of see-through). Edit mode, hover your mouse curser over each headband mesh and press L. The mesh will automatically be selected.
*Press Shift + N > Recalculate Outside.
*Press A. Alt + J to turn your tris to quads. This means fewer edges to deal with. To add edges if you need to hold Shift, select two vertices, press J.
*Open the window in the left corner of the 3D viewport and put a checkmark next to "compare UVs." Turning tris to quads sometimes messes up the UV layout and this fixes the problem though you need to re-unwrap your mesh, regardless, as it’s a mess right now.
*Open the UV editor in one of your windows (in your blend you have the image editor open but not the UV editor).
*In the UV editor, UV > Seams from Islands. You'll now see red lines in your mesh indicating where the seams have been marked. Right away you'll see problems similar to this:
Fix the seams: *Press A > Edge > Clear Seam. Edge > Clear Sharp while you’re at it.
*Edge select mode. Hold Shift + Alt and select edges to mark. Edge > Mark Seam. Try to place them where they are least visible and place as few as you can get away with. Here’s where you may run into parts where you need to add an edge back in so you can mark it.
*Some tips: Hide anything in your way with H. Alt + H to unhide. Separate with P (if you need to). Ctrl + J to join. To zoom into a mesh section, select a portion and press the Numpad period key. Press Numpad 5 to enter User Orthographic view, Numpad 5 again to exit (sometimes helps). You have some holes in your mesh. To add faces, hold Shift, select three to four vertices, press F. To isolate a mesh section, select with L, Ctrl + I to invert, H to hide.
*Edit mode, edge-select mode, select one marked edge, Shift + G > Seam. Your marked seams will be selected now. Edge > Mark Sharp.
*U > Unwrap.
*Use the edge split modifier at the end once you’ve fixed the mesh issues and arranged your UV map.
*Duplicate the mesh twice and assign cut numbers.
The trick is going to be texturing UV islands that aren’t square shaped as alpha hair UV islands are.
A few random points: Your mesh does not have weights assigned. I think the weight you need for this is b__Head__. Assign it 100%. Your uv_1 and vertex paint may need to be fixed but I’m not sure of that.
