Let me go through them. Assuming Male 50KG (lol, I pick up that weight with one arm) weight sedentary lifestyle, so around 56g of protein a day recommended:
Watercress: You'd need to eat a (non actual) ton of this to make up the protein required; Near 2.5kg of this. But that is 280 calories total for all of that, which is not fine, as you've just dumped a large mass into your stomach and have yet to obtain the necessary calorie amount. Even split by 3 for three meals a day, you're looking at 800g of watercress (that's 8 to 10 bags of the stuff from places like Waitrose and Sainsburys) PER sitting. But then you start to also: Calcium and Magnesium excess by many times the daily recommended amount. Calcium is directly bad as there are several illness' related to its excess. Magnesium is largely bad for kidneys, and all this just to maintain a daily recommended level of protein. That's not a good trade off. And certainly not for more elderly G6PD people (to which we can assume at least 80 million people of that 400 million would fall under this).
Alfalfa: Wouldn't work. Legume family based. Auto out.
Spinach: Excess Iron. Leafy Green. Remember that of that 400 million people, at least 100 million are female, and assuming everyone lives to 100, at least 45 million at around the age or passed the age of having menopause. So this is not suitable for large consumption for at least that many people.
Cabbage: Could work. Least amout of known issues for G6PD sufferers. But again suffers from needing at least 5KG of the stuff to make up the daily Protein level. So split by 3, that leaves us with at least 1.3KG of cabbage. So that's two small cabbage heads per sitting. And... well. Again, if we just put two small cabbage heads together and put it near our stomach, we can see it ain't gonna fit easily.
Asparagus: One that could work from what I know, unless if you suffer from other complications with Asparagus, to which I know my mother does (Night Leg Cramps, 100%, every time, Asparagus is involved, she suffers from it). Techincally not G6PD issue, but again, as having G6PD doesn't preclude other issues, you really need to factor them in too if they're common enough. Most Chinese people are warned off Asparagus if you have leg issues. And of course, I dunno about you, but every time my friends have Asparagus, I make sure I'm far away from them and downwind.
But I will agree this is possible, but I doubt many will want to eat to the levels required. High amounts of Iron however, is an issue for the older G6PD females too.
Broccoli: I have this already, but not to the amounts you'd need it per day for 50KG Male. You're talking near 2.2KG of the stuff. That's around 10 heads of broccoli, I don't know about you, but that's probably a bit too much space for a 50KG male person to intake to their stomach. Even if we split it down to 3 for 3 meals a day, we're looking at 3 heads of broccoli only. Put 3 heads of broccoli together and put it near your stomach, and tell me you can stomach that down, with other foods as well, since it won't give the necessary calories required alone; Again, for a 50KG Male.
Cauliflower: This suffers from the same issue as Cabbage. It'll work, but you need to consume an inordinate amount of it to make up the protein level. We're looking at 10 small heads (4" diameter) per day again, and split by three meals, and again you're looking at at least 3 of them per meal.
Brussels sprouts: Can work, other than it won't make it popular with other people
. But, the primary issue remains where you need to eat near 2.2KG a day of it. That's 1 and a half bags of it each sitting (near 700g of it). Certainly just about possible to place in your stomach. Probably the only one possible to do for daily intake really that you've listed. But women of advanced age or have already hit Menopause can't take it (as their usual method of removing Iron in their blood is now stopped).
Rice: Suffers from the same issue as Watercress, you'd need to consume near 2.5kg of rice alone to make up that protein recommendation. No one eats that much rice. No one. And I'm Chinese, I have rice, and I can barely take in 300g each sitting (and that's during large family gatherings over special occasions, that is also filled with other foods), much less 7 times more that. Or, if I just have rice three or four times a day, that's still 3 amounts less than whats required to get the required protein, and even with a large stomach you can't possibly handle that much rice (not on a 50KG Male), not without overeating by a WIDE margin. Again, the trade off is, non-ideal. Plus, whilst its silly to point out as I've not seen many Chinese people die from Arsenic from consuming rice only, but Rice suffers from having high concentrations of the stuff, which at 300g (most people eat at per sitting), now you're talking near 7 times this if you go this route. Much more research is needed, but its fairly clear it's not a wise decision.
Compare this to 1 Chicken Breast fillet. Weighing in at 172 grams. Yeah, I don't know about you, but I'd rather not try and bust my stomach to keep parity with that 172g of Chicken Breast to be honest. Compare the size of that with 3 heads of brocolli, or 900g of rice, all just to keep parity on the protein side, and not even factoring in excess of other nutrients that have now become bad for you through overconsumption without even meeting the Daily Calorie intake required.
And of course, we haven't compared the price of the amount of vegetables you need to purchase, and the fuel prices to cook that many for ONE person. And of course, needing suppliments for anything else you may miss through the combination of the possible foods above. If I had the money those atheletes do as well, sure as I'm a bloke, I can try Vegan for a while. But for a female with G6PD? Auto-fail after certain milestones are hit. So Vegan for all will never be possible.
From what I can tell, what you and
@ttaskmaster are posting is basically the same thing, but you're both looking at it from different sides. Lack of protein isn't the illness itself. It's the illness that arises from Lack of protein. And of course, as I've posted above, the amount you need to eat to retain the same parity (in protein alone) is stupidly high. So high in fact I'm sure most people will die from ruptured stomachs, etc long before any G6PD triggers can kill them. Not to mention the excess amounts of other stuff that will be an issue. So yeah, not knocking being a Vegan, but certainly not suitable for everyone as per your postulation in an ealier post (that as Atheletes can do it, everyone else can survive if not thrive off it).
Believe me when I tell you this: I've gone through ALL of what you've posted before years ago (near 20), and it hasn't changed one bit since then when I first found out about it. My mums current GP that sees her most often looks at us sheepishly as she went through the same stuff before when she took over being the primary one dealing with her a few years back. And all she's done is make further (current) notes that agree with older notes from prior Doctors (who had recommendations from Specialists from St Thomas and Guys) to not to continue repeating the same questions as it won't change the outcome.
(They know better then, until a new one takes over and tries the same stuff and then gets the evil eye from us as a "We told you so". Then promptly puts in notes again repeating the same stuff to not ask us again about the same stuff...
They're like kids that can't stop themselves from touching fire and burning themselves really because we know the next one will repeat it...
)
