First of all, welcome 🙂
Well, I haven't tried that comboy myself but the J12 is certainly a tricky lens. If you're thinking about buying one, I'd wait for somebody who has tried that before. Also, even between the same lens there are built differences between some generations, so it's a matter of trying carefully.
If it wasn't a bottom loader, I'd tell you to look through the open shutter while screwing, but I guess that if you get it you'll have to rely on 'touch' to see if the lens has collided with something inside the camera...
Have you considered a Canon/Serenar 35/2.8 instead ?