You are right with your assessment of Brazier and Ingebrigtsen, and Ingebrigtsen struggling to break 49 in a FAT 400m.
I did NOT criticize Brazier's training. I said, the reason his 1500m time is not that great and not up to his ultimate potential is because he runs very low mileage. If he ran a lot more mileage, he would undoubtedly improve his 1500m time from his 3:37, but it will also come with a cost and might lower his 800m times by 1-2s. Of course, he is doing the absolute right thing by focusing on the 800m atm and training low mileage for it which worked extremely well for him.
Lewandowski was not a speed guy in 800m, he was a strength guy like Symmonds. He did a lot of fast training for the 800m, but always got smoked by Adam Kszczot on the quick reps (he was the stronger one during threshold work). Think of Lewandowski as 800/1500 guy, and Kszczot as 400/800 guy. That's a big difference. Lewandowski needed that high mileage to get any good at all in the 800m, whereas 40-50 mpw are enough for people like Kszczot, Rudisha and Brazier, who have signifiicantly more 400m speed than Lewandowski (1-2s 400m FAT speed becomes quite significant on that elite level).
Ingebrigtsen is a current 1500/5000 guy, and later will be a 5000/10000 or even higher guy, that's why he gets destroyed in 1500m sprint finishes by Lewandowski. In an 800m, Lewandowski was the one who got passed on the home straight and yes, often was the poor 4th place guy.
What I want to say - while Rudisha, Kszczot, etc get away with their 30-50 mpw because they are clear 400/800 guys with lots of FT fibers, the only way someone like Lewandowski or Ingebrigtsen get any good at all in any race is by running high mileage since they come from the strength side. So saying Lewandowski could have been faster had he run 70 mpw instead of 100-110 mpw in base phase and 60 mpw instead of 85 mpw during competition is probably not right. Symmonds probably falls between these two extreme spectrums, but still more on the strength side, with 75 mpw during base phase and ~65 during competition.