We recently had a long discussion about online Ontario licenses. I think it is one of the 'redneck' threads nearby

Links are in that thread. They do sometimes have the Walpole licenses in stock though they sometimes ran out later in the year depending upon volume. It would be wise to check ahead anyway.
Technically, all Walpole territory is also in Ontario. I have always bought both when I intended to fish that part of the lake. I imagine you could get stopped and checked by Ontario authorities while fishing in Walpole territory? Probably more likely if you are in the main river channels or out in the lake edge parts. I've never actually seen MNR or similar Ontario authorities up in the Walpole channels.
I rarely see MNR anywhere. The few times have been up the main St. Clair River channel around Fawn Island and down on the South Shore of the lake. I think I have seen them running around near the shipping channel in the lake a time or two.