Best file format for translating a CSV of short user comments?

Hi, I'm working on a research project where we collect public comments from TikTok and YouTube videos in several languages. We export them to CSV with Comment Viewer (one row per comment, columns for author, text, likes, date and parent reply id), and I'd like to send only the text column through the MotaWord API for translation into English.

A few questions before I build the pipeline:

Is CSV a supported source format directly, or should I convert to XLSX / XLIFF first so the other columns are kept untouched?
2. Can I mark columns as non-translatable, so usernames and dates are not sent to translators?
3. Is there a minimum billable word count per project? Many rows are only 3-10 words, so I'd rather batch a few thousand rows into one file.

Thanks!