Well the idea of a DM critter implicitly assumes DM particles can interact with each other through other dark forces, but they can only interact with real matter through gravity.

The thought experiment of whether you can measure directional gravity from freely falling reference frames using only gravitation is more interesting.

>> You can only measure a gravity gradient by comparing it to another force

This is inaccurate. A measurement of tidal forces only requires the ability to measure space and time between events/particles.

>> They can't see redshift because they can't see photons at all.

Gravitational redshifts affect all clocks, not just photons: it's a statement about spacetime, not literal redshifts. All pieces of matter are clocks ticking at the Compton frequency, so DM particles can be used as clocks to measure redshift.

>> dark matter is simply energy, there is no extra particle

I feel that 'simply energy' would be something like dark energy, causing spacetime expansion. You really do need mass for gravitational attraction, and what we measure is gravitational attraction.