When being asked, what's the most harmless way to address my past record of leaving Ph.D. program in a different field?
Based on the information you provided in the question, I think you should focus on reason (b). You should probably avoid using words like "abuse" or "confrontation", at least in the beginning of the conversation, as they come off as really strong and thus tend to motivate people to question deeper. State that you have had problems with the previous environment (including the adviser and the department) and that it was in the mutual best interest that you continued your career somewhere else. Avoid blaming anyone. If the conversational partner seeks to question further, reveal details to what extent you deem best, but again it is good practice to avoid bad-mouthing your former environment. Stay as objective as possible.

Like you said, most people don't care that much. For those that do care, you can't really stop them from taking it negatively (much as you can't stop anyone from taking anything about you negatively), but you can stay true to yourself, by being honest and objective without bad-mouthing your previous adviser, department, etc.