I've found the Cyrodiil point at Pale Pass, the two Morrowind points in eastern Rift and Eastmarch, and the Hammerfell point in the southeast Falkreath... where's the High Rock crossing point? Or is there one?

It would have to be somewhere in the Reach. But I have not explored that area enough to be able to say exactly where.

Perhaps on the Solitude side of the map, start from the Thalmor embassy and work your way back towards Markarth maybe.Interesting all the same, I will give it a go later

I recall exploring from Markarth to the Sea of Ghosts looking for one, and I don't believe I ever found one.

Same here, off the top of my head I can't recall, but I have visited all the others that OP listed.

i think there isn′t maybe because of the rocks they couldn′t creat one so you gotta go by boat ,idk