I will get to Faustus’ oath soon, but in the mean time here are three bits I missed. At some point, I hope to put these all together on one page, in order, with line references to the various editions, since those are highly variable (these numbers are from Bevington & Rasmussen’s Revels edition):
On kai me on
This is not Latin, but Classical (and Philosophical) Greek. It would be spelled like this in Greek script: ὂν καὶ μὴ όν, but that’s more than you need to know to pronounce it. (Several early editions have ‘economy’ but that is just a wrong guess by a confused typesetter.) The pronunciation is easy: On Kye May On.
The words mean ‘being and not being’, the subject of some of the more abstruse parts of metaphysics (on, ‘being’ is related to ‘ontology’, kai means ‘and’, and me means ‘not’). Probably all the words except the ‘and’ (kai) should be accented. It should sound something like this.
Véni véni Mephistóphile.
This is not the same veni as in Caesar’s famous Veni, vidi, vici. That veni has a long E and means ‘I came’. Faustus’ veni has a short E and is imperative singular, a command addressed to one person: ‘come’. (One verb having two forms that are spelled the same but pronounced differently is not unheard of in English, where ‘read’ as in ‘I read books’ rhymes with ‘reed’ when it’s present tense, with ‘red’ when it’s past tense.) He’s just telling Mephistophilis to come to him, repeating the verb to show urgency or impatience: “Come, come, Mephistophilis”. The dropped S on the name is not classical Latin, but is the sort of thing Greek and Latin do with vocatives. The whole thing should sound like this.
The meaning is easy: ‘intelligence’, meaning presumably that an angel is in charge of turning each sphere, they don’t just turn by themselves. For pronunciations, it’s hard to say whether Marlowe would have said ‘intelligént-ee-yuh’ like modern Latinists or ‘intelligénts-ee-yuh’. More likely the latter, I think, and it might be more intelligible to the audience, too. Here’s my version.