Is German's present tense ever used to refer to the future? Like how we might take the present progressive tensed sentence in the English translation to mean "we are going to drink and you are going to pay."
Yes, it can.
Perhaps this is something that happens every evening -- a repeated, habitual action, for which we would use the present simple tense: "We drink and you pay".
Leider wird "We drink and you pay" als falsch gewertet. Ich habe es gemeldet.
That seems like a set of instructions or a sort of game plan. A better English sentence would be "When we are done drinking, you will pay".
Neither are a correct translation of the German sentence, though, which doesn't have a "when" and only says "trinken". Usually, the simplest answers are best.
The sentence "you're paying for our drinks" is different from "you'll be buying our drinks", though.
You are paying for our drinks = Sie bezahlen unsere Getränke.
Is the "for" necessary? Or is it also ok to say: "You are paying our drinks"?
You will be buying our drinks. = Sie werden unsere Getränke kaufen.
Is the "be" necessary in the second sentence?
The "for" is necessary here because that's the English version of "bezahlen" (which you could also, vielleicht, translate as finance or fund), and because you're not paying the drinks (you're paying the cashier, directly, or the owner of the bar, ...)
to buy = kaufen, to pay = bezahlen. "We are drinking and you are buying", as fas as I know means: "Wir trinken und du kaufst." Korrekt?