Hi there :) 

Thanks for improving the data :)

I'm a volunteer as you. Regarding your question:

> Not sure if the grocery (convenience shop) should be on the building or on the POI (is there a convention ?) 

If the whole building is used as a convenience store (including any building levels above the ground level), then the tags for the convenience store can be added to the whole building and the node (point) for the convenience store node can get deleted.

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From the existing mapping, it seems that the convenience shop is also preparing and selling coffee.

Taking into account the "One feature, one OSM element" principle https://wiki.openstreetmap.org/wiki/One_feature,_one_OSM_element  personally, I would remove the node of the cafe, and add the following tags to the convenience area (or node):

cuisine=coffee_shop
drink:coffee=yes
drink:coffee:takeaway=only (if it is takeaway)
indoor_seating=yes/no
note=Ετοιμάζουν και πουλάνε ζεστό καφέ (or something along these lines)

Having said that, you can also ask the opinion of other Greek volunteers at https://community.openstreetmap.org/c/communities/gr/68 

Thanks again for improving the daa and happy mapping :)

