Any dangers from plugging this in to a socket?

Does it need its own earth? Will it switch off if it detects a residual current? Can it handle spikes in load?

No your normal socket can be used the other way around without issues and that reverse load doesn't create a problem.

Its even reducing your load on your main line beause the energy directly flows into the next consumer.

German socket has earth, thats fine. It has protection mechanism and shuts down if it can't sync to the powergrid. The panels only produce (in peak!) 400-800 watts. Thats not an issue.