If someone "shows" a Pokemon through GTS negotiations, will that count as being seen and therefore add to the pokedex?
I don't think so no.
For a Pokemon to be registered in your Pokedex it has to appear ingame. So this means to 'see' a Pokemon, it has to be by an NPC, or through Battle.
No. The Pokemon you finally receive in the actual GTS trade will be registered in the Pokedex.
Really, any Pokemon you see in a place that requires saving is not registered in your Pokedex as 'seen'.
Hope I helped. :)