Ok, weird question..... Not sure exactly if I'm thinking of this correctly, but here goes:

If I had an Inquisitor and Retinue (which, as a group, counts as a "unit"), could I join a Primaris Psycher to the unit (since he can join units)? If so, and I have an Astropath in the retinue, could said Primaris use his Lightning Arc (shooting attack) when a unit attempts to deep strike in for the other army (for the free shot on said unit that Astropaths entail)? I'm not sure if I'm making any sense, and I'm not sure if I remember the Inquisitor Retinue unit status reading, since the codex is at home.... Any thoughts on this?

If it's a Power used as a shooting attack you can use it. I found out the hard way when my daemons came through a warp gate and got caught between some grey knights and an Inquisitor riding in a bane blade.

I guess I should have figured that if he was in the unit he could shoot away.... my issue is I don't know if he can legally join the unit.... I guess if I have 2 astropaths, I can give the ability to him that way, but what I was thinking was if he could join up with the unit, I could save the points for a second Astropath and just join him into it