Why do the same letters in English have radically different pronunciations in different words?

There are, not so much rules, but tendencies for why letters are pronounced so crazy-different in different dialects of English, and so differently from Early Middle English.

Unfortunately you need to go through a lot of historical phonology to make sense of it. Fortunately Wikipedia has a decent summary of both the historical phonology, and of subsequent changes. Unfortunately you need a linguistic background for the changes to make sense to you.

Phonological history of English

Basically start by assuming that the spelling used to make sense, and then move forward from Middle English for each of the sound changes. (The changes from Old English to Middle English aren’t as critical to making sense of the spelling, since spelling was reset from scratch after the Norman invasion.)

The craziness of vowels around l’s and r’s, in particular, is something very characteristic of English—and it makes sense once you realise that, compared to other European languages, l and r are pronounced quite back in the throat (retroflex and velarised)—which makes adjacent vowels be centralised.

Like I said, you need a linguistic background for the changes to make sense to you…

What impact did Crete have on Ancient Greece?

As Toby Williams said, significant in pre-Classical Greece—after all, the Mycenaeans got their writing system from the Minoans, and there are echoes of the old Cretan dominance in the myths around Crete.

In Classical times, not much at all. A couple of philosophers (including Epimenides and his paradox), but Crete was a backwater. That continued even into Byzantine times: Andrew of Crete  is all I can think of for cultural impact on Byzantium. It was only under the Venetians that Crete had a significant cultural impact on the Greek-speaking world (including art as well as literature).

How often do you have to write formally and with proper grammar at your current job?

I’ve switched careers from being a humanities academic to being a business analyst. Currently I’m more a data analyst with a sideline in IT architecture and policy. The clarification is important, because business analysts are more human-facing than data analysts.

I switched from natively writing in Dickensian paragraphs, to natively writing in dot points. My audience would much rather read something created in Powerpoint, than something created in Word. I learned that there was not that much point correcting simple spelling mistakes from colleagues (though that’s not as much of an issue in my current position!) I relearned something I had learned before linguistics, when I was interning as an engineer: I am writing for people who would rather not be reading what I am writing, so I don’t get points for eloquence or cleverness—just concision.

I still happen to write formally, because of my training (although I use less semicolons than I used to); but I notice that few of my colleagues or their bosses bother. I have one boss who is a stickler for formal (and impersonal) tone, but I strongly suspect this is a generational thing. In fact, I’ve been in the middle of an edit war between bosses on formal tone of a document. I like the subjunctive, but have found that it outright confuses colleagues; then again, the subjunctive is now mostly an American affectation in the Commonwealth.

All that said, precision is valued in my job, so “proper grammar” is appreciated.