Sealing a stained cutting board

Yes. The extent to which that is "sealed" is debatable, but there's certainly no issue with finishing over cured stain - it's done all the time.

Just stay food safe (and be sure to check with your guests about nut allergies if using something like walnut oil, which is a good food-safe hardening oil, but not great if you have a guest with tree-nut allergies at a very sensitive level...)

My dad used to seal all his handmade cutting boards with mineral oil. What I noticed was an odor which I didn’t care for. MOST mineral oils are petroleum based.
I found and use food grade board oil, as well as conditioner. The odor is much more palatable. AND- since I have a huge number of food allergies, much safer than nut based oils.