Hi Karsten - that's an excellent idea for a Danish place of origin for "Splan." I like it!
DeTails, you seems pretty committed to this idea of a Danish connection, and Karsten and you both got nibbles when casting your nets for a Danish "Splan" place. So I'm yielding a little and thinking maybe the connection to Denmark does exist. In that case, it can't be too far back or it would have dwindled from family memory.
All you can do DeTails is trace each family member back, one step at a time.