You wrote:
“The Carpatho-Rhatians descend from a Cossack group sent into quell a rebellious new possession of the Czar.”
Who? I think your conflating the Carpatho-Rusyn with the Rhaetians. They actually live very far apart.
His version of events that led them to live far West of where they'd originated is consistent with what you read on the net.
NOTE: I cannot be responsible for anomalous spellings that have their origins with individuals doing their own transliterations from Russian written in cyrilic characters to a Western Slavic tongue written in latin characters.