Generally speaking, Birkat Kohanim can/is done at any daytime prayer. We do not do so at mincha because in the afternoon people have already eaten and the kohanim may have drunk some alcohol which would invalidate them from performing their duty.
Musaf is usually prayed in the morning after Shacharit; however, one is permitted to pray Musaf lechatchila until the end of the seventh hour, and bedieved all day.
My question is, do the kohanim perform Birkat Kohanim at Musaf if it is being prayed after noon? What if it is being prayed after Mincha?
Finally, if the answer to the above is No, does that affect saying Sim Shalom vs Shalom Rav in the individual's Shmoneh Esrei? Generally speaking, we (Nusach Ashkenaz) only say Sim Shalom at a prayer where Birkat Kohanim could be said.