I haven't fully read through this entire thread, but here's my suggestions.

First, get a medical evaluation.

Sounds like you've eliminated at lot of the usual issues...

Have you tried the gelatin PT? It works LEAGUES better than anything else I've ever tried. (For me at least.)

I add EVOO to my DT's, but you can try an other oil - perhaps EVCO or Grapeseed? I also add honey and a little ACV. (I also add EVCO and ACV to my gelatin PT's, too.)

Here's an odd thing you might try. My hair is pretty porous, and it was always dry. I used a lot of LI and sealed, but it didnt last the day.

After reading a couple articles, I tried mineral. It made a HUGE difference!!! A few drops on very wet hair over my LI - WOW! Helped seal out humidity in the summer, and I find it holds moisture in well now, too. In fact during these winter months I've been using a few drops of MO and a pea-sized bit of vo5's hairdressing. The results have been great!

Those are just some things I would try.

I second the clarifying suggestion, and the maybe follow with a PT and then a DT.

Olive and coconut oil are two oils that penetrate the hair shaft, so they are great for adding to DT's!

I have even clarified, PT'd, then DT'd overnight by putting a bag on my head and a sock hat over that, and then rinsed in the morning. (Yes, it was a great look. Ha!)

Double check all your products to make sure no cones have snuck in there. And remember that if you get your hair cut they might use something with cones, too. But cones aren't the only thing that can build up - I like to clarify every so often anyhow.

Good luck!!
2c/3a/3b, Fine/med, Dense, high porous, & normal elas.

Lo-poo - Renpure Keratin, SMR, Giovanni SaS
clarifying - mix Johnson's H2T with low poo
Co-wash - Suave Nats. clar. or coco.
RO - GVPCB, Ion Eff. Care, TN.
LI - SMR, Cure Care, or RO
Seal - MO or Jojoba
DT - + honey, ACV, & EVOO to SM masque & GVPCB
PT - Gelatin!, CNPF
Styling - FSG, AIF, BRHG, LALS/PS, GF curl spray gel, SM Souffle/Smoothie, LOOB
Oils - MO!, EVOO, jojoba, GSO, SM Elixer, EVCO, VO5 Hairdressing