I'd connect the optical cable from the cable box to the sound bar. Sometimes when going from the TV you won't get a 5.1 signal because the TV reports back to the cable box that it is only a stereo device. So it is usually better to connect it to the cable box instead.
yes disable the speakers on the TV.
Cable box should be set to fixed, you want to adjust the volume with the sound bar and not have two volume controls (cable and sound bar)
Regular speaker depending on setup and models will probably sound better than a sound bar in general. Sound bars are typically convenient, sound better than TV speakers and can offer simulated surround effects but typically for sound quality regular speakers will sound better. Again depends on your speaker placements and speakers themselves.