Thank you for the all the effort you've put into this. I'm still having the same problem as before. Before the second MAC change, I get internet access both on ConsoleNintendo3DS and my Wi-Fi connection that is being shared. When I click on the Nintendo Zone app on the 3DS, I get through with no errors. I didn't get any Streetpasses, though. Following the second MAC change, there is no internet access. I get "Error code 003-2005" when I try to use the Nintendo Zone app.
Try to change thw eireless adapter driver. During the start the MAC is not changed, so with the native MAC internet is accessible for you, but on the next mac change something happening wrong, most probably it is a driver issue.