Hey alefteris,
a) I removed the POI and added it's tags to the building, I changed the "amenity" tag because I wanted the symbol to appear on the map and I believe that using the tag "police" is not "tagging for the renderer" beacuse the coast guard in Greece has many similarities with the police. If you think this is not appropriate please change the tag to "coast_guard".

b) Leukimmi port is not an industrial port, it serves passenger traffic and a small amount of cargo traffic. 
"Ports or parts of ports serving passengers, without cargo traffic may require a different tag such as commercial=port as such areas may not qualify for landuse=industrial" 
See more info: https://wiki.openstreetmap.org/wiki/Tag:industrial%3Dport

c) For the road classified as a secondary highway is because this road links the ferry route to the National Road 23, although changing it to "unclassified" or "service" I believe wouldn't make much of a difference.
"Use highway=secondary to tag highways which are not part of major routes, but nevertheless form a link in the national route network" 
See more info: https://wiki.openstreetmap.org/wiki/Tag:highway%3Dsecondary

Let me know your thoughts and thank you for commenting.

