Sorry for the delayed reply. I've had severe brain fog and mental fatigue along with a bug with my first account not allowing me to post. So I just created another account.
@Jodie:
MCAD Classification:
I was confused by the classification at first too. I've seen publications where MCAD and MCAS were considered synonyms. But it's confusing to have a general term be the same as one of it's subtype, so I prefer to use MCAD for the top level or most general term. I think my MCAD classification page can take it from here:
evilmastcells.com/MCAD-Classification.html
MCAD & Impaired Detox:
As how impaired detox could relate to MCAD, I basically thought the same as you. If chemicals were more slowly metabolized then it seems higher doses would pass from bloodstream into tissues making contact with hyper-reactive mast cells. So I think that may translate into a lower concentration and/or duration of environmental exposures needed to provoke a symptomatic reaction than otherwise. Also if mast cell mediators such as histamine were more slowly metabolized then that would make us more sensitive too. I'm not sure how significant a role it would actually play in MCAD.
However I'm concerned about people with MCS trying to "detox", particularly when they think feeling worse is a good sign which they call herxing. Instead I think they're actually having a reaction to the detox treatment putting them at risk of becoming more sensitive.
MCAD & Genetics:
I think MCAD is like many other conditions where there's a genetic susceptibility along with environmental factors to initiate a symptomatic disease. Many MCAD-associated genetic mutations have been found in mast cells involving mast cell regulation. Almost all mutations in MCAD were somatic instead of germline, but it appears to run in families leading them to believe it may have an epigenetic component which was supported by a recent study. Epigenetic changes control expression of genes, turning genes on and off, which is affected by environmental factors. And epigenetic changes can be inherited. The variability in mutations affecting mast cell regulation are believed to be a major contributor to the huge variability in MCAD manifestations.
Specifically they've found many genetic mutations that alter the KIT receptor, a major mast cell regulatory receptor. The most well-known is D816V mutation found in the majority of systemic mastocytosis (SM) cases. The D816V mutation causes the receptor to be constitutively active, no longer requiring it's ligand to bind to the receptor for activation. So the mast cells are receiving an autonomous, excess signal from the mutant receptor to proliferate and to survive much longer, resulting in accumulated mast cells (mastocytosis).
So this KIT receptor is a drug target in SM, and there are drugs that inhibit the KIT receptor, called tyrosine kinase inhibitors, such as imatinib, helpful for a minority of SM cases, but the D816V mutation is resistant to it. But I just read about a new potential drug tested in vitro that does inhibit the D816V mutant KIT receptor.
Of course SM is a rare MCAD, so most with MCAD will be more interested in MCAS, but I just wanted to give an example that once they understand the biochemical causes of the hyper-reactive mast cells in MCAD we can expect better targeted drugs to follow that are super effective. I think they will fall under mast cell stabilizers. I think it will be many years yet though. But increasing awareness of MCAD will probably increase funding for MCAD, so I'm trying to do my part.
I've barely read on the genetic component of MCAD so far, so I can't give a complete summary yet. I'll probably have to study immunology & genetics and get some ADHD meds to read more on this and understand it better. lol I do plan to learn a lot more so I can eventually explain much more details in the future.
New MCAD Book:
Right now I'm reading a new MCAD book: My Crazy Life- A Humorous Guide to Understanding Mast Cell Disorders. I've been in the mood to read MCAD stories. And it has a section on disability which I'm in the process of applying for now. It also has a section on weird triggers & symptoms. So the table of contents lured me in.

I've only read a few chapters so far but I can say that people with MCS could definitely relate.
@Maff:
I think other hypotheses of MCS, if based on findings in MCS patients, may be other aspects of MCAD because MCAD can affect so much biochemistry in the body. For example, histamine can potentiate NMDA activity, and PGD2 is reported to be one cause of brain fog in MCAD. But it's also possible there's other mechanisms of MCS besides MCAD that are as different as type 1 & 2 diabetes are to each other.
Antihistamines are the first-line therapy for MCAD, but they don't help everyone with MCAD. Mast cells can selectively release inflammatory mediators and the types and doses they release vary per person causing different symptoms and requiring different drugs to address the different mediators. Although mast cell stabilizers can help prevent several types of mediators from being released.
Some tested for elevated mast cell activation didn't have elevated histamine, but had elevated PGD2, and some of those had enormous benefit for symptoms such as brain fog with aspirin which inhibits PGD2 production, but aspirin is commonly not tolerated in MCAD.
However, it can be difficult to detect the elevated mediators so if elevated histamine isn't showing up on tests it doesn't mean antihistamines won't help. Tests aren't good enough yet to guide treatment much; it's mostly trial and error of meds that may work.
Years ago I took the NMDA antagonist dextromethorphan which seemed to help my brain fog a lot, but the brand I used apparently is discontinued and all the others I've seen have inactive ingredients that I doubt I'd tolerate.
There's still a lot I need to learn. I wish I had enough mental energy to research many of the other MCS hypotheses and see how they may relate to MCAD. But I'm also not very motivated to read much MCS studies because I'm not confident they selected the MCS participants carefully enough to ensure they actually even have MCS, which may be a major contributor to negative results in MCS studies.