According to surviving documents, the Karen/Qarinvand dynasty being a major player in 867 is very sketchy. "The Minor Dynasties of Northern Iran" chapter in The Cambridge History of Iran Vol. 4 has some relevant info, which I'll quote below:
"Tabaristan now came under Tahirid rule for over two decades. Quhyar, who had been promised by the victors possession of the kingdom of Vindadhhurmuzd, was killed by his brother's Dailamite slave guard, and it has been generally assumed that the Qarinvand dynasty came to an end at this time. This assumption is probably wrong. In events of the year 250/864 an Ispahbad of Lafur, Baduspan b. Gurdzad, is mentioned. He and his descendants through three generations are occasionally referred to until 318/930 as rulers of Lafur, Vinda-Umidkuh and Vindadhhurmuzkuh. One of them is designated in a contemporary source as a Qarinid ("Ibn Qarin"). After 318/930 the dynasty apparently declined to insignificance and is not mentioned for over a century. But towards the end of the 5th/nth century Ibn Isfandiyar again mentions amirs of Lafur and expressly calls them Qarinvand. It is thus evident that Baduspan b. Gurdzad also must be a Qarinid, though his exact relationship to Mazyar is unknown.
The Bavandid Qarin b. Shahriyar, who also had aided the Tahirid conquest, in reward for his services was restored to the rule of the Sharvin mountains. In 227/842 he accepted Islam." (pp. 205-206)
TL;DR, there are recorded figures in Daylam who are referred to as "Sons of Qarin," and if we assume that these were Karen/Qarinvand folks, they retained some land between the late 9th and early 10th centuries.
Needless to say, the Karen/Qarinvand dynasty controlling such large swaths of land in 867 is pure fantasy.