I often travel to Xiamen. XMN is a rather small airport. It now handles a lot of international flights. But the airside area is not that large, so they don't let you enter many hours before a flight.
The staff does not speak a lot of English but I found everyone friendly and immigration fast. Getting a 24h transit visa should be reasonably fast and simple as you are using XiamenAir on both legs. And delays are much less at XMN than PVG or PEK coz flight paths are less subject to ATC congestion. They were talking on being able to issue boarding passes at the departing airport for a through itin on XiamenAir (not KLM), but I don't know if that is operational. Anyway you will have to recheck your luggage.