They're marching to within effective range, im unsure what weapons they are using here (probably the brown bess), but the average range was around 50m, so that's where they're marching to, they march as a unit to keep cohesion, and also so that they can be controlled and manoeuvred more easily.
For the same reason above, so they can get into range, it would be a waste of ammunition, and time, to fire from beyond this.
Normal Regiments of the line would be rarely used to flank an enemy, this would normally be left to cavalry or another unit to do. If a flanking was about to occur, this particular Regiment would be tasked with fixing the enemy so that they would be unable to effectively engage a flanking unit.
Unless it was a well placed shot to the torso or head, most people would survive a volley, albeit with wounds. If your Regiment had been wiped out in a way as portrayed in the video, you'd want to make a rather swift exit from the killing area as the enemy may advance towards you with the Bayonet.
This was a rather basic answer, but any further questions and i'd be happy to help!