I use a Rainbow Adapter with my Minolta MC/MD lenses. It is solid aluminum, and seems sturdy. The lenses kinda lock in place, but can be twisted off without pressing the release button. There is, though, enough friction that the lenses would never fall off. It also allows me to focus beyond infinity, but so do the NX lenses in manual mode... As for worrying about a heavy lens damaging my camera, I doubt any of my manual lenses are as heavy as the NX 85mm f/1.4... Besides, at some point you stop holding the camera and start holding the lens instead.
Having said all that, I love using manual lenses. Sure they are heavy, but they are a great value! I have a 28mm f/2.8, a 58mm f/1.4 and a 135mm f/2.8, all for $90. And since I also have a Minolta 35mm camera (Full frame!) it's like having a set of six primes: 28, 42, 58, 87, 135 & 200mm.
I used to wish Samsung would release more fast glass at a variety of focal lengths, but now it does not bother me so much.