"Since this system hasn't ended yet, and it's lasted longer than a literal generation as it is commonly defined, and since the scriptures say that God cannot lie....how would you define 'generation' in Matthew 24:34?"
I would answer this with "I would continue to define this generation as a literal generation and say that generation hasn't arrived yet." Keep it simple. If this were someone that I wasn't related to I would be sarcastic about how my mind is blown at the idea that you can talk for more than 6 paragraphs about how the definition of a word can change four or five different times and still not have it right.