Theres also the issue of his being shocked by the brilliance of the kid in question. Why would he be shocked by someone raised by Callista?
Raised by Callista doesn't mean anything if you see the kid not meeting your expectations.
If you enroll your kid in the best school but see him getting D-, would you think he's a brilliant child? Would you not be surprised by your D- kid scoring a scholarship, no matter where he went to school or what tutors he had? Especially if you had evaluated him as D- yourself & had great confidence in your judgement in that aspect??
He is shocked that he couldn't see that briliance before, that someone was able to fool him & hide their true self from him. This has nothing to do with Callista. Olivia's acting talent should have helped with this, fooling the great Alexander Karlsson

Only Olivia <3<3
As for secrets, the whole game thrives on secrets. Alexander had loyal servants/allies. So did Callista.
He had the power to arrange the whole Gambit in secret. Even after he's dead, he's still keeping his secrets. I don't doubt his ability at keeping things secret...
who is sneaking her on and off the island for these meetings
Also, what island? The game so far takes place on one island, but the whole Karlsson territory was his playground. They could meet at any place, multiple facilities, instead of smuggling Olivia into an area with heavy board presence. Like the temple in the flashback, it had Alexander's building but was open to MC, hence not on the "island".
and if he wanted Olivia so badly, why would he allow her to stay with Callista?
Working theory would be he wanted her trained & raised by Callista as well as himself. After all, we still don't have much info on the gambit or on Alexander's motivations/intentions. Or Callista's for that matter.
yet not one person appears to know anything.
Olivia also seemed to not know anything, but the 4 flashbacks from ch6+7 say differently, with each chapter showing she knew more than we previously thought.
The Alexander connection is a guess, sure. But Olivia knowing more than previously thought is pretty much confirmed, either about the Karlssons coming, from the chopper flashbacks; or in more detail about Alexander's mindset & their family blood relations secrets, from the hiking flashbacks. Question is how far does that go & how much she knows...
As for veronica, we can assume that veronica and probably yvette knew of Callista and Kane since he was the kid from the earlier chapter when they first met.
I meant about the whole Olivia situation. That connection was already revealed pretty early, (that they had contact & communication with Callista, & Stacy even thought MC was lucky for having her as mom when she identified Callista's fav quote), then used to obsfucate the grymgudinna identity.
But yeah, the current working relationship between Veronica & Callista would be interesting to think about, given grymgudinnalives establishing a similar relationship between Alessandra & Callista - a relationship we previously thought was between Veronica & Callista
But yeah, the Alexander connection is based more on possibilities & following the trend.
It's more prediction than anything.
Olivia knowing however, either about seth or herself or about the gambit, etc., is pretty much confirmed from the flashbacks tho.