[ It's a significant history in Istredd's eyes, but that is more to do with the importance of the moments they shared to him. The monoliths, the coup at Thanedd, the protection he's had for Ciri on behalf of her parents. There is a separation in his mind between people from their sphere and those he meets here. Although his friendship with Geralt only exists here, it would not have happened in their sphere. And he values it.

He feels valued in return when Geralt shares because he knows it is only on Geralt's terms when he does, and opening up even a small amount matters. A part of him understands the pleasure he derives from being let in is unique to Geralt too. Istredd has a thing for withholding people giving him just a little. ]

It's unlikely. The vision has already been proven incorrect due to Sidwell's coup. But perhaps you can convince him to leave the guard. I would, were a loved one to request it.