I can't really see how he is defending the design by sending you files. Even if he created it entirely from scratch it's still virtually identical which makes it infringing. You say 75%, I'd say closer to 95%.

Going forward, would this concern be alleviated by not releasing the solidpart files and just the STLs? It would certainly make any duplication much much harder to perform. The other option would be to sell the v2 design through your store front exclusively for 3-6 months and then release it as open source after that time period. At the end of the day I'd think that the goal would be to give you more freedom to design instead of wasting your time tracking these people down.