Hi John,
It's possible Rockne played later under an assumed name later on. Many players did that in the early days of pro football ... particularly if they were either college players or college coaches.
Jack Cusak was more than Thorpe's friend. He was the Manager of the Canton Bulldogs and orchestrated the signing of Thorpe in 1915. He wrote a small book/pamphlet on his time at Canton that a lot of the more recent articles / books are based on. I have a copy and it documents his years at Canton.
jeff