I wonder how many times, especially as a teenager, Zhao Yunlan was summoned into his father's presence and greeted with "You've made me so disappointed."

From what I've picked up here and there, this seems to be part and parcel of Chinese parenting? But even if not, I agree, he's heardf this hundreds of times.

At what point does Zhao Yunlan realise Zhao Xinci is not himself? Is it in the car and he bides his time to make the challenge, or do his suspicions crystalise when he hears Zhang Shi talking to Guo Ying?

I think in the car he might be mildly surprised, but that it's not going to be official, but it's what Zhang Shi says to Guo Ying (particularly about the SID) that makes him sure this is not his dad.

This scene always strikes me as evidence that however much Zhang Shi claims to have been a sort-of-parent to Zhao Yunlan and was there for his childhood, he can't have been actively involved or said much -- otherwise his speaking now (and on the roof during the kidnapping, and in the scene during the blindness arc) wouldn't strike Zhao Yunlan as definitively OOC.

Absolutely. I always feel that Zhang Shi might have taken Zhao Xinxi's odd lapses in vigilance here and there to surface and actually for once be nice to the kid, but that happened rarely and did nothing but give ZYL emotional whiplash. :(

I wonder how differently Zhao Yunlan would be reacting if a large portion of his brain weren't busy screaming with worry about Shen Wei.

Interesting thought. I felt he was keeping that well locked-up here in order to function as he normally would, but who knkows...