Gravity waves are caused by moving objects, and the waves themselves move. So if you did see constructive interference between gravity waves, it would be temporary. Gravity waves also tend to be extremely small, small enough that it was only this year (2016) that we were finally able to detect them (at LIGO).

Here's a video of what standing waves look like in water: https://www.youtube.com/watch?v=NpEevfOU4Z8

Ah, so 'Gravitational waves' are ripples in spacetime.

But, if gravity itself propagated as a wave (graviton + duality?) could interference explain dark matter/energy?

Gravity, in its normal sense (of matter's gravitational attraction), has no frequency or wavelength. Its only property is that of acceleration.

That's right. Quantum mechanically, potentials are generated by the exchange of off-shell mediators (the clearest explanation I have seen is in Zee's QFT in a Nutshell).

https://en.wikipedia.org/wiki/On_shell_and_off_shell