Bone demineralization would not be detectable via bloodwork. It would require a DEXA scan. The body keeps calcium levels pretty constant in the blood and will rob it from the bones if necessary because it is such an important mineral for many metabolic processes.
Low B12, low iron stores and low D all scream of celiac disease as opposed to NCGS! You would not have those deficiencies with NCGS because NCGS does not damage the villous lining of the small bowel. Of course, some or all of those deficiencies could be due to other medical conditions but the fact that you can't do gluten sure points to celiac disease. I do understand that you rely on the NSAIDs for pain control but they are also known to be hard on the gut and I wonder if it is the cause of the intermittent bleeding you mention. Concerning bariatric surgery for weight control, I have know a number of people who had that done and none of them were happy with it in the long run. It can create a number of other health/nutrition related problems and when you already are starting out with some significant preexisting medical issues I would be skeptical of it.
Seborrheic dermatitis can definitely be frustrating, especially when it causes itching and redness. You’re not alone—many people with celiac disease or gluten sensitivity also struggle with skin and scalp conditions, and some find that gluten exposure worsens inflammation. Have you noticed any connection between your diet and flare-ups? Some folks see improvement with gluten-free or anti-inflammatory diets, while others benefit from medicated shampoos (like those with ketoconazole or selenium sulfide). It might be worth tracking symptoms alongside your diet to see if there’s a pattern. Hope you find some relief soon.