“Hence, there were no Republican slave masters at all.
Perhaps an overstatement, as it is likely there were some in the border states. However, they were most definitely a tiny minority.”
Not sure about that. There were many black slave owners, as well as native American slave owners.
Neither group was allowed to vote at the time.
The GOP was founded as explicitly against the spread of slavery and in favor of its eventual extinction. This was a quite unremarkable position for slaveowners in the early years of the country, and indeed was the position held (in theory, anyway) by Washington, Jefferson, Madison and most of the other leading slaveowning politicians of the time.
By 1856, however, positions had hardened and for a slaveowner to join the GOP he would have had to be willing to call down on his head the universal anger of his compatriots.