In the sentence 「これまで僕はいろいろな人と言語交をして。」Why was これ used instead of 今 here? As far as I know まで means something along the lines of “up until” so in order to say “up until now” why say 「これまで」and not 「今まで」？ Also, would 「今まで」be grammatically correct in this instance?
I'm not a native speaker, but my feeling here is that it is basically like saying "up to this point" in English. That is, it is talking about where they are in their progress along some path (of learning/conversation/etc), rather than really talking about "this moment in time".
You could say 今まで, but I think this would have more of an implication of "I have been doing this (continuously) until just now". これまで has more of a feeling of "I have been doing this to get to this point/situation/etc".