You have got it right, and it is not a paradox. A "strictly non-commercial" product does *not* qualify as "open" (see for example the [Open Knowledge Definition](http://www.opendefinition.org/okd/) paragraph 8), because you are discriminating against a field of endeavour.
The CC-BY-SA license that we currently use does not allow you to place additional restrictions on the data. A "you may only use this noncommercially" restriction is, in the eyes of this license, none better than a "you may only use this if you pay royalties to company X" restriction.
The CC-BY-SA-NC license, which does have a non-commercial component, is not compatible with CC-BY-SA (see [compatibility matrix](http://wiki.p2pu.org/w/page/12427319/License-Compatibility-Matrix)), and is not considered an "open" license.
OpenStreetMap plans to switch to another license, ODbL (see [OpenStreetMap Wiki](http://www.osmfoundation.org/wiki/License/We_Are_Changing_The_License)), which will allow you to license derived products differently; under ODbL, you may place a non-commercial restriction on a derived product (unless that product is itself a database).
Note that for your particular use under today's license, if you do not intend to publish the wall map, or if you can publish the OSM and non-commercial components separately and have the end-user combine them, the license restrictions would not be irrelevant. relevant. They only come into force if you publish the combined product.
