Hi Justy,
Hope you got on holiday OK and that your stomach didn't let you down too much.
I was diagnosed with gastritis and duodenitis around 10 yrs ago. In my case I did have an endoscopy and they didn't find H Pylori from taking a biopsy. I have since had a blood test again on this last year, and that was negative again.
At the time I had not been diagnosed with ME/CFS and I didn't know what I know now about PPIs, so I happily settled down to taking Omeprazole for 2 yrs, before I decided that I didn't want to spend a lifetime on it. The Omeprazole did in the short term deal with my gastritis but not in the long term.
I then started to research as I know you have, all my foods, and what else I could try. Clearly my gut issues, probably like yours are complex, and still highly sensitive though I have it under some control now. I now use Betaine HCL but just one capsule with each meal though I think if you have gastritis badly that needs to heal first maybe. I am not sure but instructions seem to indicate not to use if you have an ulcer.
Someone mentioned DGL liquorice, which i did find useful.I know I did a lot of research and cabbage water came out highly, but I could never face it! Also the GAPS diet recommend bone broth and I think that is very healing. I know that you like me have tried the kefir without success in the long run.
My foods have had to be severely restricted for both IBS and gastritis reasons.....obviously everyone is different but for me I can now only eat unprocessed meat/fish, fresh veg and fruit, nuts, seeds, raw chocolate, and coconut butter/oil and coconut milk, and not a lot else if I am to have a happy day!