This is an interesting thought! But it's impossible for me to picture James as a medical professional; his character doesn't work for me if I think of him as someone who would have encountered serious illness on a day-to-day basis before Mary became ill. We also at one point hear him saying, 'But you’re a doctor! It’s your job to heal people! How can you just let her die?' in a flashback conversation, which I can't imagine he'd say if he worked in the medical field himself.

Also, I think you'll find the manual clearly states his occupation is protagonist.

(Apparently the creators have said he's a clerk. I assume he works as a clerk part-time to make room for his protagonist duties.)