Hello Laura,
this is a late reply to your post. I hope in the meantime you have found help and some answers and are managing to cope with your pain.❤️ But maybe you are still searching and trying to find help.

Having suffered of chronic neuropathy pains all over my body, and now - after years of spending time and tons of money consulting all sorts of doctors and therapists (I have finally been diagnosed with non-length-small-fibre-neuropathy and spasticity) I am aware that I can only help myself.

I totally understand the urge and longing to find someone - just someone - who will completely appreciate your situation and condition and offer real practical guidance and support. However, I am afraid the best we can do is to learn as much as possible about ourselves, listen into our bodies, our pain triggers and what might help us and give us relief.

I am trying to put myself into your shoes - it must be terrible not being able to sit without pain. I suppose lying down does not give much relief, or does it?

I had never heard the term arachnoiditis before, but did a bit research. Attached some quick info FYI. But I am almost positively sure you know all this already (although the dietary recommendations might still be useful). Sorry, if this is too simple and basic.

I also stumbled over something called spinal cord stimulation. Here is a link to one of I am sure many "providers".
https://www.acmcrn.org/post/spinal-cord-stimulators-for-arachnoiditis-weighing-efficacy-risks-and-new-technology
Since you mentioned you suffer from spasticity as well, I have found a highly interesting paper on spasticity and vibration therapy, also attached here below. Over the years trying all sorts of therapies I disvovered that high-frequency vibrations trigger my pain drastically, but low-frequency vibrations like gentle rattle-shaking combined with mild stretching "unwinds" my muscles and softens the pain. It is just so hard here in Europe to find a low-frequency vibration plate.

This paper focuses on comparing different frequency vibrations. Sounds promising and I hope there will be more appropriate physical therapeutical therapy possibilities soon!

Hope this helps a little!
I will be thinking of you❤️
Good luck and be well!