Somebody else will probably explain this more eloquently by the time I finish typing but here goes:
As to the first statement, that probably refers to moving the shoulders in front of the haunches so that the horse can be truly straight. Horses are wider behind than in front and are naturally crooked, so if you let them follow the rail their haunches will tend to fall slightly to the inside. This isn’t quite the same as riding shoulder-fore all the time, IMO, because in shoulder-fore you should have more bend to the inside whereas if you are riding straight down the track ideally your horse will be straight. According to Walter Zettl, in the shoulder-fore a horse should be bent as though on a 20 m circle (vs. 10 m circle for shoulder-in).
That said, I do find that the idea of shoulder-fore helps develop straightness, especially in canter and especially to the direction in which your horse’s haunches tend to fall to the inside. I ride my GP horse in right-lead canter with the idea of shoulder-fore to keep him straight, but if we are traveling on a straight line there is no bend.
As to your second point, you want bend to correspond to the horse’s line of travel. So, you want bend on circles (more or less, depending on the size of the circle), in corners (more or less, depending on the training level and ability of the horse/rider to ride deep into the corners), and in lateral work (except for leg-yield, where there should be flexion but not bend). Bend should be from nose to tail so I do think the image of the horse wrapping around your inside leg is accurate as long as it’s done evenly and the haunches aren’t merely falling/pushing in. If the horse is going straight down the long side or across the diagonal, you want them straight. (But referring back to point one, riding them straight still means keeping the shoulders in front of the haunches.)
Does that help at all?