Back in the days my mother was also very anti-MSG and probably still is. I was also wary, mostly because it even has to be stated by restaurants when a dish contains it etc.
But then I actually looked up what it was and was like "why are people even debating this, how did that happen?!".
These are the 21 building blocks of ALL proteins of ALL life on earth (don't start with the 22nd now please
), the common amino acids:
See that glutamic acid on the top right? MSG is that with an ion (Sodium) to make it a salt you can put on stuff:
(the lower one is when it's used as neurotransmitter in your brain)
Not to mention the studies that there are no harmful effects and basically all of Asia using it just fine. Both to be expected when it's something you are literally made of...
Well, yes and no. Everything you said about umami and what MSG is is true. However, some people do gave bad reactions to MSG, and it is a common migraine trigger, along with other significant naturally and non-naturally occuring sources of glutamates, such as tomatoes, mushrooms, aged cheeses, et al. I can only speak to it as a migraine trigger, but it is a common one, and as such I don't think it's a delusion amongst those suffering from migraines, even if the whys of it aren't well explained. When you start getting migraines, your neurologist will often recommend you remove all common migraine triggers from your diet, and then try to slowly work them back in to see what's a trigger for you personally.
I can't however speak to its veracity as any kind of allergen, and I understand there is a proven nocebo effect in studies showing it isn't an allergen. I can only speak to it as a migraine trigger. People sensitive to such triggers aren't allergic to the foods, but just as ingesting a chunk of a delicious aged Manchego can be a migraine trigger, so are commonly many other foods, such as MSG and similar products such as yeast extract, modified corn starch, etc.
Virtually the only thing I could imagine being harmful about MSG is an extreme overdose (in the sense that anything can have toxic effects in high quantities) and if there is a high amount of impurities during production (e.g. D-form instead of the L-form), which both might lead to unexpected mild side effects in very sensitive people.
As you said, allergy is non-factor at least if we talk actual allergies (your immune system attacking something, in that case a molecule found in every part of your own body).
The migraine trigger seems interesting though, do you know if there are studies for it? I'm not saying it's bullshit because I guess depending on how your body reacts to (suddenly) readily available glutamate, it might fuck with the balance of it as neurotransmitter (potentially switching some synapses on that shouldn't be).
Edit: googling seems to indicate that yeah, glutamate buildup at the synapse (not due to MSG in that case but if it were able to get there the effect would be the same) might trigger migraines.