Have you tried something like a mouse sander. They’ve always gotten into nooks that my larger sanders couldn’t. Otherwise a soft touch with something like a wood chisel might work.
Looks like a nice pallet for staining! Should look really nice when finished.
You could use a cheap detail sander like this. I have this one and it has worked well for me. The tip is removable so there are two triangular shaped tips for getting into places like this.
BLACK+DECKER Mouse Detail Sander, Compact Detail (BDEMS600) https://www.amazon.com/dp/B00OJWLNMO/ref=cm_sw_r_cp_api_glc_fabc_doa7FbXQVY59R
I tried a sanding sponge with a super coarse grit too. It’s like nothing touched it at all. It’s a hand held sander, like this - BLACK+DECKER Mouse Detail Sander, Compact Detail (BDEMS600) https://www.amazon.com/dp/B00OJWLNMO/ref=cm_sw_r_cp_api_i_qXoHFbXG0EH66
Ok, I have done a lot of testing with Dremel's and 3D printed parts. My best conclusion is that you are far better off with some kind of oscillating sander instead of a barrel or a disk.
I use this one it comes with a weird little finger extension thing that goes on the front that helps get into narrow places. (you can see it on the 5th picture)
Drum or disk sanders on a dremel just generate way too much heat and end up squishing plastic around and generally making a mess of things. Yes with high grit on a super low speed you can get a decent finish, but it takes you 1/4 the time to do the whole thing with a sander like the one above.
The one thing that I do use the dremel for is friction welding some parts together. Makes a really good looking weld if you are trying to replicate that look. Just put a piece of filament in the chuck and run it between the parts.
Not sure how it compares to the similar, slightly cheaper models on Amazon but I have the B&D Mouse and it works fine.
https://www.amazon.com/BLACK-DECKER-BDEMS600-Detail-Sander/dp/B00OJWLNMO
I've had it for ~3 yrs with no issues, although the dust collector is basically worthless.
I'd have stained it (cheap and easy to make it look a lot fancier than it actually is) and then apply 3 or 4 coats of minwax tung oil to seal it up.
My wife and I inherited her grandmother's old dressing table and mirror. It was in awful shape. We brought it home, I bought a little mouse sander (seriously amazing little guy and the precision tip can sand tough to reach crevaces) and we stripped it down to the bare wood, stained it with some minwax red oak stain and a few coats of tung oil and it seriously looks like a really expensive thing now.
Well if you're at the point where you're considering repainting the headstock, I'd try sanding it off first if the toothpaste and alcohol don't work, using a sander like this to avoid gouging the wood: http://www.amazon.com/BLACK-DECKER-BDEMS600-Detail-Sander/dp/B00OJWLNMO
You'd want to sand it before you painted it anyway.
Not really sure what your budget or needs are, but a palm sander (also called corner or detail sander) might work for you. They are relatively inexpensive and the sanding pads are too. I've used a Harbor Freight one almost daily for many years for many different types of projects.
Here's a decent one from Amazon
Whatever sander you decide on, an essential add-on is a gum rubber sandpaper cleaner. I went for years always replacing the sandpaper when it got clogged until I discovered that I could clean them over and over, and save a lot of money.
Hope that helps!
Deal link: Amazon