1) Yes and no , depends on what you are cutting. "Toothy" low grit edges have large teeth , when these teeth break off the plateau where the apex used to be will be quite wide , going from sharp to dull quickly. When smaller teeth break off a much more refined edge , the plateau they leave behind will be much smaller , this feels like a sharper knife than the degraded "toothy edge" and will stay in that middle zone of acceptable sharpness for longer. But they will be less durable doing certain tasks.
But overall I feel that edge geometry and steel type play more of a role in edge retention than finish does. Heres the kicker , to touch up a more refined edge is much easier than touching up a coarser edge , and removes much less material. If my EDC has a 0.1u finish , then to touch up I typically either just use strops (0.25u , 0.125u and 0.1u) Or if I the edge is really degraded then I go up to the 30k shapton (0.5u) and correct any damage and then go back to the strops.
2) I haven't found it yet. And I play with stuff finer than 0.025 micron.. Leaving a coarse edge hogs off the life of a knife in a hurry as you touch up and do repairs overtime.
3) You can stop wherever you feel like. If you like the performance of the 1u edge more than the 600grit edge then go for it. If the edge is not holding as well as you would like tweak the geometry a little bit.
FWIW an 8k edge should start looking like a near perfect mirror , if its looking hazy or anything like that then you probably need to do more work down low before moving up.