Yes, quick search is OK, but if "Find references" feature was released - it should work fully OK, else you cannot debug fully correctly.
It's just MHO, not a requirement...
That would be nice, but I can't see how it's even remotely possible. Consider that Callsub can call using variables, not only static strings in the call, and all of a sudden it's a surprisingly complex problem to find those references.
The only realistic improvement I can see is when the callsub uses a static string, or when it's using a variable that is set using known data in the same sub. Anything beyond that and I imagine it's quickly getting insanely complex (and impossible).