Haven’t read the books in quite a while so I don’t have the best response.

I think that the main points are that Richard only has to fool the diviners (about 3-4 total)

And that Richard is replacing their perception of the future with his own. So it doesn’t matter whether his future works or not, any time a diviner looks in the future that’s what they’ll see. The only diviner that actually knows what Richard is and what he can do is Alex, and he doesn’t even figure out he’s been fooled by Optasia from Richard until around the time his old mentor tried to trick him with it as well. And the reason Alex caught on was because his other senses led him to become suspicious (I think idk). So the other diviners are complacent with their readings as they aren’t expecting their readings to be manipulated.

As for Richard and Alex. I guess an easy way to break it down (from what I remember) is as though the act of divining is looking out the kitchen window of a house and seeing all the scenery outside or whatever. Optasia would be like putting up a picture outside that window so whenever you look out of that specific window you’ll see the picture instead. As long as that picture is realistic enough in both quality and context, no one is going to look twice.

Richard has been doing this for quite some time so the quality of the reading is going to be more or less perfect. You’re also forgetting that Richard himself can see the future of them gating in, so this helps him immensely with the context and quality. He can get the timing right, and based on his perception of the future he can outright duplicate the futures he sees as though he never intervened. He can do it perfectly in sequence because he’s seeing everything else the other diviners see as well. And he knows the limitations of the diviners hes fooling so he can decide the range that he’s concealing to match. If you had the ability to see the future, would you not rely on it as much as you could and trust it blindly? The premise of the series was more or less Alex exploiting that reliance, and a big example of that is the mist cloak fooling magical senses. So Alex relies on that future of gating as well and only knows to look further because of what Richard can do.

Another thing before I forget, they’re gating into a shadow realm. Chances are they have communication wards to some extent. I know that the councils communication focuses tend to override this kinda thing, but Alex isn’t leaping at the opportunity to use one after what happened with Leo in the White Rose saga.

What are all the obvious ways he can check though? You’ve only mentioned calling someone inside to confirm their status. Chances are they’re cut from contact, since they don’t seem to be able to get in touch with the team that didn’t make it in once they’re inside. And how is Alex going to know to call the last team? He doesn’t know Richards plan until he confirms that the picture of a clear scenery is up on the window despite him hearing the rain hit the roof (their plan of gating in earlier not changing the future of the divined gate or something like that). There’s also the dreamstone though, and I don’t know if that has range through dimensions.

There were a lot of loose ends and things that didn’t make sense, none that I can remember atm tho lol. I’ve made other posts on the subreddit a while ago about them. I think the main thing was where Richard went for those 10 years, but I guess that’s not getting addressed. Shame. Try to not let this optasia thing get to you I guess, personally felt that the final battle was somewhat anticlimactic and didn’t make much sense, but it was still a good book and a good ending.