There were a lot of parts to the original question, but I'll focus on the part(s) about how to handle separated parties and compartmentalized knowledge.
Here I've learned a lot from
@iserith, who unfortunately seems to no longer be part of this community (not surprising, considering the savagery of the responses to his contributions). In a nutshell, I make zero attempt to police players on this. If Character A gets into trouble, and Character B's player wants to go rushing to help, I don't have a problem with that. I could ask them to rationalize their decision, but I leave it up to them to offer that rationalization if they want to.
The only real objection I ever hear to this approach is "but that's metagaming!" (Using the specific/narrow definition that it means conflating player knowledge with character knowledge). But if you don't really care about that, then it's not a problem.
(Also, in my experience, some/much/most of the time Player B will roleplay ignorance and not go rushing off, anyway, even though they know that it would be totally kosher to do so.)
A related objection would be something about the believability of the fiction. But in my opinion that's a pretty predictable, mechanistic definition of believable fiction. Unlikely coincidence appears in good fiction all the time.
The last objection I can think of is that splitting the party is supposed to be a risky decision, not to be taken lightly. But there's no guarantee that Character B will actually
get to Character A's location easily or quickly. And rushing there might cause even more complications. (E.g., now they are BOTH in solo combat! Yay!). I don't know exactly what those complications might be...it would depend on the scenario. But, yeah, splitting the party is still a meaningful decision. And really that's all I care about: players making meaningful decisions.