When looking at the Dragon Dieppe Naval Infantry soldiers, I noticed that they look largely like World War One sailors, with such obvious differences such as guns, hats, insignia, etc. with peacoats and dark trousers.

Why World War One sailors? They of course rebelled immediately at the end of World War One and caused the German civil war that followed.

In Germany it was never called a civil war, The Kiel munity (what you´re referring to) which triggered the "November Revolution", was as you could say pretty peaceful...
Altough the "Spartakus Riots" a few moths later weren´t...

The Kaiserliche Marine used what is known here as the sailors siut, and this is looking a bit different from the uniforms in the Dragon offering.



But if you´re willing to do a scene from the "Spartakusaufstand" you could easily use one of the Volkssurmsturm offerings.
and that for both sides, governmaetal troops were sometimes also just in civil cloths, those were them miainly part of the "Freikorps"

so, long coats were always fashionable
for soem pics check this: https://www.google.com/search?q=spartakusaufstand&source=lnms&tbm=isch&sa=X&ei=sYjqUuTrBMGctQaT6oG4BQ&ved=0CAkQ_AUoAQ&biw=2021&bih=915&dpr=0.95