I've been reading a lot about INS and have a few of questions that I hope someone can answer.

First of all, when the INS is slaved to the auto-pilot in the real world, is it coupled with the heading bug like on the sim, or is it coupled with an HSI (which seems more logical)

Second, could INS only go direct to way points, or was it able to track courses?  For example: Let's say you're departing an airport on a Standard Instrument Departure and you are initially getting radar vectors.  Then shortly after departure, ATC gives you a vector to intercept an airway and says proceed on course. ls it possible to intercept a specific course using INS, or is it only limited to direct to?

And finally how did aircraft maintain accuracy with the INS on trans-atlantic flights?

Sorry for the long post, just curious.