Can this be made as a layer cake? If so, what size? What frosting / filling would you recommend?

I agree that yes, it is possible to make this as a layer cake. If it were up to me I would use orange curd as a filling and dark chocolate for the glaze/ frosting. ;)

I don't see why not. The baking time/temp will likely need to be adjusted though. My go-to round pans are 8" which I usually bake at 350F for 30 - 45 minutes depending on the batter. Flavors I like with orange are vanilla, cardamom, pistachio and dark chocolate. Good luck if you try it!

I think you could (as a thick 2 layer, 8 inch cake or thinner 2 layer, 9 inch cake), but be careful that any filling or frosting doesn't dominate the flavor of the cake. I would use a very light buttercream, maybe flavored with orange juice like the glaze, and put a touch of buttercream and some orange marmalade between the layers. Actually, I think this would be really good as a naked cake--plus then there's less to frost!