You know there are some pretty good names on your list but at the same time that list kind of proves my point.
Guys like Grba, Burnside Kemmerer, Palmquist, James, Casale, Daniels and Kutyna were likely guys who would not have been in the majors if not for expansion and Roger hit 12 against these guys.
Then you have better pitchers who were over the hill and perhaps expansion gave them time playing that they would not have gotten otherwise, such as: Conley, Fornieles, Nuxhall and Wynn and that is another 5 home runs.
Subtract those 17 and perhaps add back a bunch because he still would have hit some against other pitchers and it is pretty easy to say that Roger would have had had a great 1961 season even without expansion but his HR total might have been more like 50.
Where I can not find numbers is the interesting thing (interesting to me only probably) that Maris, Cash and Gentile were pretty similar hitters and perhaps the 3 hitters who benifited most by whatever was happening in 1961.
You know, Roger jumps from 39 to 61 and then drops to 33 homeruns and people wonder about that but the most amazing differences to me is that of Norm Cash's batting averages.
He goes from 286 to 361 in 1961 and never hits over 280 again in a long career????????????