Shakespeare coined (or at least first wrote down) hundreds of neologisms presumably just to make the verses scan. His English was never the best, honestly. Like, in terms of being normal English. Much of his poetry was creatively odd, but also grammatically odd at the time. He sounded unusual even to his contemporaries. That's part of why he made such a big splash.

why "the" but not "a"? Same meter.

But I agree it's silly that say "the" is what makes Macbeth creepy, and not, you know, the occult theme that permeates it.

Stressed "the" and unstressed "the" have different implications for meter. (And meaning.) "The" has two pronunciations in English. "Thee" and "thuh". The former is stressed, the latter not. While part of it follows the same pattern as a/an governed by by the initial sound, some of the rules are complicated. "Thee" is also used for emphasis as a demonstrative.

"Give me thuh cat toy." (Some ordinary toy.)

"Give me thee cat toy." (The one with special powers.)

But how was it pronounced at the time Shakespeare actually wrote Macbeth?

If you listen to a performance of Shakespeare that replicates the original pronunciation, it's wildly different.

I'd wager the "thuh" pronunciation is a modern simplification and not something Shakespeare accounted for.

https://www.youtube.com/watch?v=YiblRSqhL04

https://www.youtube.com/watch?v=qYiYd9RcK5M

(though, having watched the second video, the actor clearly uses "thuh" and considers it Original Pronouniation, so perhaps I'm mistaken)

The pattern of the articles the and a/an being affected by the sound of the following word dates to Old English. The use of the stressed form as an emphatic does as well. It probably goes back further. There are similar traits in the other Germanic languages. German routinely reduces its definite article die ("dee"), to approximately "duh", and its indefinite article ein ("ayn") to approximately "uhn", as well. Except when emphasized. We even have traces of emphatic "a" in English which is now completely archaic -- except still irresistible in "an historic moment". That probably counts as a fixed expression now. But etymologically speaking, when someone says that they're saying "one historic moment" and emphasizing its distinctiveness. Germans would say "ein historischer Moment" and it's a safe bet they'd say ein (one) and not the usual "uhn" (a).

Sure, but the article doesn't attempt to classify the stress, and the stress is often flexed as a convenience to help the meter or to avoid hard -to-pronounce double vowels, not to change emphasis.

"I don't what that one; I want thee other one. Thuh yellow one, not thee orange one."

In BrE, "the" before a vowel is pronounced "thee". So I'm not sure what you're saying.

Precisely. 'the' is an easy single-syllable choice to pad out your meter, so it makes perfect sense that it was abused here.