月 and 肉 initials are much closer in Cantonese which is why Cantonese speakers often fail to differentiate these sounds in mandarin. I don’t think it’s directly linked to the pinyin, which has a different evolutionary path.

You see the same in Nanjing dialect, where speakers fail to differentiate l/n initials and n/ng finals. The pinyin is different, but their home dialect doesn’t distinguish so they fail to distinguish the sounds. Presumably if Nanjing speakers had been writing the pinyin standards there would be more chance that lan/rang would have ended up written the same.

https://en.m.wikipedia.org/wiki/Nanjing_dialect

I think what you’re after probably is a historical analysis of romanisations including pinyin. Such an analysis might also cover other no-obvious features of pinyin, like why jiao and yao have similar vowel sounds but different spellings and the nv/yu and nu/wu similarities and differences.

For your historical English example, I think in some dialects and in some historical times there would have been people that fail to distinguish some of your examples and others but all of this is a bit beyond my expertise.