Excellent comment. Whilst I'm no scholar of languages I'm aware of some of those matters to which you refer so my earlier comment is somewhat rhetorical.
I became curious about diacritics decades ago when learning French at school. At best, I was only an average student of languages including English, so I found French quite a challenge—my strengths were in the science subjects and if I hadn't been pressured by my parents it's likely I wouldn't have studied it (my mother has French relatives thus I have French cousins and my father, a mechanical engineer, whose native language was English studied both French and Latin at school and later acquired some German—hence the pressure). In hindsight it was a damn good thing I was pressured as it made learning other languages later much easier (a modicum of German and Latin—learning German was out of necessity as I'd lived in Austria for some years).
My French pronunciation was pretty abysmal and grimaces on my teacher's face during oral French were testament to that. However it improved after diacritics came to the rescue, I then found it easier to pronounce words such as être, café and Rue Taherè with some degree of accuracy (Rue Taherè is in Paris where my French relatives once lived, thus it was essential I pronounced the grave correctly).
Sorry, but that long intro was necessary to justify my point that those who are not particularly good at languages and or spelling can benefit from the use of diacritics. At the beginning I didn't know how to pronounce those French words accurately but diacritics greatly simplified the process. What was particularly good about them was that I didn't have to remember the various pronunciations of 'e' across dozens of different words, that luxury isn't afforded to learners of English! French was hard enough with its gendered nouns and such without that additional burden. It's why I pity those who have to learn English as a second language, the lack of diacritics is just one awkward aspect of this 'tortured' language that makes learning it so difficult.
One thing I've learned is that those who have a natural aptitude for languages and spelling and who develop fluency easily often fail to appreciate how really difficult they are for many people and I include native speakers here.
Given my struggles with French I thought I was pretty dumb when it came to learning languages but my view changed a little some decades ago after I attended a lecture at the Australian Museum in Sydney given by Noam Chomsky. The notable linguist made an unexpected (and what I thought was a rather extraordinary) offhanded remark that I'll never forget which went something to the effect:
"I am not good at acquiring languages, a new language can waft down the corridors of MIT and it can completely pass me by only for me to later find that my colleagues acquired it with considerable ease—much to my chagrin."
What Chomsky taught me was something that I was already subliminally aware of, which is that people can acquire language skills in quite different ways and those who acquire them with ease do so in ways different to those of us who have difficulties (it's why they often don't understand why some of us find learning languages difficult).
I learn languages like mathematical equations, I atomise them bit by bit which means I want exact literal translations of foreign words and phrases before I'll parse them into idiomatic/vernacular English. Those who are good at languages don't do this, they learn to go directly from the foreign language to their native vernacular sans atomization. Atomization is a mode of thinking and it comes from a reductionist mind. Whilst experience has taught me that reducing languages to atomized bits is not conducive to learing them it's nevertheless very hard to kick the habit.
"The Great Vowel Shift came along and fucked up all the long vowels, and separate changes caused "a" to break into 2-3 sounds..."
This, I reckon, is still very relevant to present-day English. Whilst only part of the problem it accounts for the peculiarities we now have with, say, 'gh' words such as cough and though which I've mentioned. Though they've morphed from their earlier guttural sounds their spelling hasn't yet caught up. Combine, this with the many other influences on English, Anglo Saxon, Norman invasion, etc., etc. and it's little wonder the language is in a complicated mess.
I don't have any specific proposal about how diacritics could sort this out, I'm only suggesting that it would make sense if we English speakers spelled [spelt] words the way we pronounced them, say, somewhat akin to German.
Of course, attempting to force change on a language is doomed to failure, it seems the only way is for it to evolve. The 'z' and 's' spelling differences between US and British English are already controversial enough without trying to change, say, you to just u, or through to thru. And that's just spelling, imagine the difficulties of trying to force through grammatical changes such as changing the second person you to youse or to the archaic thou even though it would make sense. Incidentally, my father's old German textbook translated the German second person du as thou. I've not seen this done in any modern textbook but it's an almighty powerful way of embedding the notion of never committing the faux pas of using du and that sie must always be used unless of course it's very clear the situation calls for it.
What I find annoying is that few Anglophone speakers care about these mstters. Unlike the Académie française, (which although essentially ineffective, it at least acts as a reminder), there's never been any authorative oversight of English (and with its dubious history it's doubtful that any oversight would have ever been possible). Nevertheless, without it English has been left to self-styled people like Noah Webster to make improvements, however without any formal structure in place there's little to propel ongoing effort forward after they're dead. Murray, OED et al have done an excellent job at documenting word meanings and their spellings but they've had little impact on actual word usage. Moreover, it also seems that dictionaries have little impact on word usage in orher languages, words only get added to dictionaries after they've taken hold in a language, not before (I recall a once Dutch girlfriend of mine making this exact point about that remarkable Dutch tome—the Woordenboek der Nederlandsche Taal).
In essence, whilst reformers such as Webster have done good work and have done so with good reason, in some ways they've made matters worse. For instance, the fragmented spelling caused by differences between British and American English is already a nuisance and counterproductive in today's global world. Yes, it's perhaps only a small matter in the grand scheme of things but it's indicative of how difficult it is to make changes to a language, especially so English. Whilst, say, German has had minor reforms to its spelling and punctuation in recent decades (the German orthography reform of 1996), such reform is hardly applicable to English and can't be used as a test case for a number of reasons. For starters, German has changed very little over hundreds of years, and with a little effort modern-day German speakers can read texts going on a thousand years old but obviously that's nigh on impossible with English without special training.
Moreover, the tortuous history of English has left us with an almost incomprehensible English grammar. By that I mean it doesn't have 'clean' rules, it's much messier and less consistent than say Latin, or French, or German, and I'd suggest that there's no one who comprehensively understands all of its rules. I own a copy of The Cambridge Grammar of the English Language which, going on 1,900 pages of fine type, is testament to the fact. I've often thought this book should always be hidden from prospective learners of English for fear of giving them apoplexy: https://en.m.wikipedia.org/wiki/The_Cambridge_Grammar_of_the....
If you read that Wiki then it'll be obvious that even with such a well-researched book about English grammar written by experts in the field that there is still noticeable disagreement amongst them (see the section headed Reception). These disagreements back up assertions about inconsistencies in the language.
I was intending to comment on your point about words imported into English as it's an important aspect of the problem but the post is already too long. I'll think about it and perhaps I'll post it as an addendum.