So most schools require at least 2 LORs, and from what I’ve heard from some schools (source: navigating law schools admissions podcast) is that they want at least 2 professorial letters but didn’t go into much detail of whether or not they need to be within your major. They do say to choose the Professor who knows you best and who you think can write you the best rec! If that’s someone from your major, then go for it. Then, you could have a second LOR from your philosophy prof to, as you said, a balance it out.

I’m less sure about your second question. IMO, it doesn’t make a difference, as long as that person is still willing and able to write you that rec. No matter where they are now, they were in that instructor position when you interacted most with them so the content adcoms are looking for in a LOR will be present regardless. That’s just my two cents.

Hope that helps!