As Harald points out, flat gravestones with an inscription along the edge and a human figure in the middle is a common medieval type. I don't know what would make the style of this stone "Celtic" unless it might be the "triangular" shape, or perhaps the style of lettering.
I couldn't find anything about that in any of my books on medieval art and iconography. If it's true, I would be very interested in reading more.
However, we don't really need to know whether the style in Celtic in order to know how to analyze a question like this one.
It's a relatively straightforward problem of looking to see whether there is a statistical correlation. How many and what percentage of gravestones in this period are this type in (a) Hebrides, (b) Jämtland, and (c) Skancke family. Then ask whether this type of gravestone is more likely to appear in the Hebrides than in Jämtland, and whether the Skancke family are more likely to use it than other Jämtland families.