Thank you for your question.
It is quite likely that all these signs are interrelated.
Her choice of overgrooming site is one that can be seen with a range of issues. It tends to be that 'easy to reach' place (ie inner thighs, belly, tail, etc) that cats will target when the have issues with allergies (ie food, environmental, pollen, etc) or stress. This is what makes it hard to tease out the trigger for her signs on history alone. (As opposed to flea allergy dermatitis causing very characteristic signs at the neck, tail base, and back).
Now in regards XXXXX XXXXX dander, it is quite possible that it is a sign of poor skin/coat health
Often we will see coat health compromised by a deficiency in essential fatty acids (EFAs). EFA's are the fats that are part of skin cells composition and play a role in their health and coat health. They are volatile fatty acids and can therefore may be low in commercial cat foods (especially when cats are fed dry that has been open for a bit).
To supplement them and cover bases, a general recommendation for dietary supplementation with essential fatty acids is based on supplying 1.5-2.5 ml fish oil for a 4kg cat. Alternatively, you can offer a small volume of fresh salmon weekly. If she isn't keen on fish, then you can speak to your vet about alternative EFA therapies (ie Viacutin, Yumega, or even Allerderm spot on EFA treatment). It can take a few weeks to see results from these (since it takes time for the EFAs to get into the skin cells and affect the coat) but this can aid skin health and address the dander present.
In regards XXXXX XXXXX initial skin issue, it does sound like you have been ruling out differentials appropriately. As long as you are sure that the diet trial was exclusive (no treats with allergens to counteract your hard work), was performed for at least 4-6 weeks (the time it takes for allergens to leave the cells they are tormenting), and didn't happen to contain one of her trigger allergens, then food allergy may be less likely in her case.
As well, you noted that it started in the summer, settled with the winter and is now back to being an issue. Depending on your location in the UK, it could be a hint of a pollen or outside allergen being to blame. Therefore, it is worth discussing what allergens tend to be an issue in your area with your vet .As well you do want to double check to make sure her signs are not linked to a carpet powder/laundry detergent/etc that might be in the house and possibly triggering her signs.
Otherwise, it sounds like you are not ruling out stress differentials at the moment. If it does settle with this treatment (+/- a pheremone treatment like Feliway ), then this would confirm that this is an issue here. As well, it might be worth studying her environment now as well to see if you can identify a trigger for stress overgrooming (ie any neighbor cats bothering her or sitting in her yard. Perhaps gone for the snow but back now with the warming of weather?) If you do find that stress is to blame, then often removing the trigger (if possible) or maintaince on these medications can stop the overgrooming.
Finally, in regards XXXXX XXXXX hair regrowth, there is no secret to addressing this. The reason for the hair loss is her overgrooming not a problem with the skin or hair. Therefore, its her grooming behavior that you need to address. As long as you don't feel that she is not itchy/irritated with it (which we'd have expected to slow if not stop with her anti-inflammatory injection) then you need to break the habit she has formed. To do this, the most straightforward way is a buster collar from the vets or pet store. (If the area had been higher up then baby t-shirts are sometimes an option). It would prevent the licking, let the skin settle, and allow hair regrowth.
So, there is a lot to consider for Coco. And often it is a challenge that requires us to channel our inner Sherlock Holmes (since learning to speak cat isn't possible). But it does sound like you are on the right track and being very thorough. Therefore, I'd continue as you are but consider the EFAs for her dander issue and taking steps to prevent her grooming the area to allow the hair to regrow withour her harrassment.
I hope this information is helpful.
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Her coat may be glossy but if there is dander then it does raise questions about underlying skin health. And if we were to put skin health and EFA content to the side, the other causes for dander are lack of grooming to that area (though we'd expect coat matting then) or there is a mite that we can see this with called cheyletiella (example) which hopefully isn't present on top of everything else.
You are right that stress induced overgrooming can be a challenge. It isn't a case of eliminating the behavior itself (unless you can pinpoint the cause and it is something that can be removed from her life) since it is a side effect but helping her to cope so that she doesn't feel compelled to continue this vice (its like trying to get someone to stop chewing their nails when they are upset).
Therefore, in that case, your anti-stress treatments are appropriate and a buster collar can help break the habit and spare the areas she is overgrooming. Otherwise, if you do want to try something on the skin to deter her then you can try a chew detteran spray like Bitter Apple (example). That said, do know that cats are not keen to have anything foreign on their coats or skin. And that may mean that if she is really driven and fastidious, she will groom it anyway just to get the spray off her skin. So it is worth a try but most often a buster collar is required to halt a cat from overgrooming an area.
I do think the Bitter Apple is worth a try for her and hopefully she isn't too stubborn a lass about having it on.
Otherwise, in regards XXXXX XXXXX vs. dander, it can actually be difficult to tell the two appart without a microscope. This is because the dander from the mite is acutally secondary to the mite mucking about with the skin. If they are suspected, then the vet just needs to pop a sample (which can be obtained by skin scrape, tape or combing out a dander sample from the skin) under the microscope and look for the mite (example of the tiny mite under the microscope).
I'd assume that the vet doesn't think the mite is present but I cannot say whether the overgrooming of the belly/legs is actually related to the dander on the back or not. (since there are no rules with cats to say that they can only have one issue at a time). And because of that, while skin issues are often related, it is quite possible that the belly/legs issues are secondary to stress but the spinal dander could be something else entirely.
You are very welcome.
Best to you both,