I'm off the diet now. Hypochlorhydria was definitely the cause of my diarrhea but I wasn't able to reliably stop it with betaine due to apparently needing a variable dose and every time the diarrhea comes back my sodium drops. The year completely off carbs seems to have fixed my gut disbiosis - I'm not getting the painful bloating from carbs anymore. I'm slightly towards the constipated end of things but that's probably from the inulin fibre I've been taking to ensure I'm bunged up - I've been phasing that out and will have none today.
I was trying to eat fairly low carb still (under 100g) but that's just resulted in a voracious appetite and I've put on about 4kg, all on my stomach, in just a couple of weeks. Starting today I'm aiming for 150-200g of carbs with similar protein and fairly low fat to see if that helps at all.
I had my water deprivation test which was a very clear positive for DI but it turns out the only time they have someone in that hospital who understands the test is when I'm there. They fucked it up, probably came close to killing me and are refusing to diagnose or treat me. Fortunately I had an appointment with another hospital and while she doesn't understand DI either, she's at least treating me even if she does want to repeat the test. They've also done an MRI but I'm waiting for the report. I'm seeing a pituitary specialist next week so should get a diagnosis then and will hopefully start replacing some of my other hormones (I've got no oestradiol or testosterone and lowish cortisol plus most of the symptoms of low growth hormone).