How could this happen?
Anyway, it seems plausible to me in some ways. Jutland isn´t very easy to defend from Germany, and the kings that ruled Jutland at that time weren´t that powerful.
The islands would probably not be invaded I guess.
In any case it would affect the viking-era a lot. Huge part of the actual population in Scandinavia at that time lived in Scandia/Denmark area. The majority even.