Regarding perceived or real favoritism, that isn't necessarily a result of unschooling. My sister and I had very different relationships with my grandparents because they had an easier time finding things to connect with me about, whereas she was more stressed out by having visitors/visiting. With my own kids, I try to find things for them to do with their grandparents that both will find enjoyable.
With one grandmother, that looks more like going out to lunch at a restaurant or an afternoon out at the zoo. It's more "neutral" than either of our homes and there's a clear set of expectations socially for the kids, and also kind of a natural time limit on the visit.
For the other grandmother it might be something like taking the dog out for a walk and then blowing bubbles/doing chalk in the backyard. I prepare the kids ahead of time by letting them know things like we're going to leave the electronics in the car while we're here. Or maybe I let the grandparents know ahead of time that so and so isn't feeling very social today and is bringing a show to watch while we visit.
I try to get a feel for what stresses the grandparents out, what they enjoy about the grand kids the most, and then trying to set up our visits to include more of those things and fewer things that might cause conflict.
For example the kids there were some stressful moments with the treatment of the croquet set on our last visit, so before we come over next time I'm planning to ask if it could stay put away for the visit, and I'm going to let the kids know before we go that we won't be playing croquet this time but we WILL be decorating cookies.
Sometimes it helps to reassure the grandparents that it's okay to "spoil" on something in particular. Like maybe with food I might mention that the kids had a good lunch before we came, so no need to worry about making sure they eat a balanced dinner before dessert. (No need to include particulars on what that good lunch was).
And I'll coach the kids ahead of time on things like, "grandma loves to give you a second bowl of ice cream, but she is happier if you don't ask for three bowls." And the kids know there's more at home, so it's not a big issue.