Will a SFC being placed back into a Plt Sgt slot (in which they already have ~40 months experience) make them less competitive for promotion?

a good mix of Leadership and staff positions with a heavy lean towards leadership position is almost always good. Commanders have a tendency to use strong leaders in these positions to keep forward momentum and to keep minimal performers from getting these positions and doing damage. My personal choice/goal has always been to stay in leadership positions when I was able. When I made MSG I served as a 1SG for 8 years out of 11 TIG. The only reason I held that position was because of my reputation, I was also selected for CSM but declined for personal reasons. Staff positions are a plus but I missed troops leading even if I considered myself fair. Your NCOER's in these positions are what hurts or helps you in addition to your reputation. Enjoy the Platoon Daddy time, one day it will go away and you will be on a staff cursing the day you went there :)

I don't think it will hurt your career, but it won't really help it either. Right now the focus is on soldiers with a experience in several areas. I you look at the new NCOER, you will see them talking about "Broadening" and that's because there is a real push to get you to do things like becoming a Drill Sergeant, Recruiter, instructor and so forth. It's not just about staff positions, or leadership positions anymore.

Depends on their CMF. In general, I found the most competitive Soldiers were those working in positions above their current rank. If you are competing for Master Sergeant, then it is a stronger file if you are succeeding in a Master Sergeant position.