Well, ordinarily, the alienating party is included in reunification therapy with the intention of changing their attitude toward the alienated party, which will obviously support the subject child in reunification therapy. It's less effective if there's still an alienating party lurking around causing problems, which is why they are often included.
Your situation is potentially a bit tricky. Yes, it most definitely is something you can mediate about, but I'm concerned about what happens if mediation fails, and I am inclined to think that it will fail if the psychologist won't assist with making the arrangements they're asking you to make. Obviously, the grandmother isn't very friendly toward you, so why would she comply with anything you request?
The psychologist really needs to be the objective third party here and the voice of reason that determines how this process moves forward. Indeed, that's literally their job. Perhaps you should put the pressure on them to assist by at least making a report of some description that you can present to the grandmother for her consideration first, and failing that, to the Court to get the orders changed - which, mind, is going to be nearly impossible without the psychologist's support, though there may be another way.
Can you tell me a little about the mother's reappearance? Did she participate at all in the Court proceedings? Is her time with the children stipulated in the orders? What's your relationship with her like?