Many of these old MacIl..... names seem to have gone out of fashion early in the nineteenth century for some reason. Old Parish Records show several names beginning MacIl.... which are no longer found in Scotland. My wife had an ancestry line MacIlmorrow, all of her family suddenly changed their name to McMurray in the early eighteen hundreds. I was contacted by a Lemore from the USA who claimed descent from the same MacIlmorrow ancestor. At first I could not see any connection but then I found a grave in Dailly kirk yard with the name MacIleimorow which appeared to confirm that some of the McIlmorrows had in fact become Lemores. Similarly my wife's paternal line, McConnel changed to McConnell between 1851 and 1861. I suspect the same will have happened in your line through changes in spelling in successive generations, so yes McInstalker may well be the same family as McIlstalker.