This question has three pretty significant parts, and we're only supposed to address one topic per post. I'm going to address the "meter" part of your question, and I'd encourage you to post the other parts separately for responses to them.
Macbeth speaks in blank verse—unrhymed iambic pentameter—throughout, as do all Shakespeare's nobility (at least, most of the time). This is the meter most common-sounding to us, and our ears can be easily "trained" to expect it from the text's overwhelming use of it (all nobles speak in blank verse). Let's look at Macbeth's very first line in the play:
So foul | and fair | a day | I have | not seen |
Each "foot" (called an "iamb") consists of two syllables, one unstressed followed by one stressed (I have highlighted each stressed foot in bold font). If you say his line aloud and tap your foot with each bold syllable, you'll begin to hear Macbeth's speech pattern. You can see another example of this on line 91 of the same scene:
And Thane | of Caw | dor too. | Went it | not so?
This meter is easy for our ear to follow; it is rhythmic but not obtrusively so. We recognize its rhythm but it doesn't intrude into our thoughts—until, that is, something changes.
The Weird Sisters' speech is dramatically different. They speak in trochaic tetrameter, a meter that has four feet (called "trochees") per line, and each foot consists of one stressed syllable followed by one unstressed. See their famous spell-casting scene, act 4, scene 1, when they chant:
Dou ble, | dou ble | toil and | trou ble;
Fi re | burn, and | caul dron | bub ble.
See how their speech actually starts on the stressed syllable instead of the unstressed? This makes them sound more menacing and aggressive—we notice their rhythm a lot more than Macbeth's—and then their frequent end rhyme (trouble and bubble, here) adds to their otherworldliness and creepiness. Their meter differentiates them from all of the other noble characters and adds to the mood as well as their own characterization.
In this same scene, just prior to Macbeth's entrance, they say,
By the | pri cking | of my | thumbs,
Some thing | wi cked | this way | comes.
O pen, | locks,
Who e | ver knocks.
The first three lines here are truncated, shortened by the final unstressed syllable, as though to signify the witches' expectation of Macbeth's arrival—it builds anticipation. Then, in the final line above, just before he enters, they switch into iambic pentameter (his meter)! I like to read this as further evidence of their deception of him. They want to make him feel secure, and they deceive him into feeling confident that he is safe, and their one-line switch into his meter seems like a small metrical sign of their intentions. Pretty cool, right?