I don't know if this has been discussed before, but what if Jacob's visits with the losties are not flashbacks but instead it's him changing the timeline. When he visits Kate, he prevents her from getting in trouble and he tells her not to steal again. Because of that visit, Kate becomes honest and a good girl.
When he sees Sawyer, his pen was running out of ink so he would have put that letter away. Since Jacob gave him a pen, he continued to write which resulted in his uncle seeing the letter and asks Sawyer not to finish that letter. Because of that, Sawyer doesn't finish the letter and moves past the loss of his family and doesn't follow in Cooper's footsteps of being a con man.
Jack is operating and freaks out, so his dad gives him a technique to calm down. Jack feels embarrassed by this and is upset with his father. When Jacob gives him his candy bar, he suggests that it just "needed a little push". Obviously he was talking about more than just the candy bar. Because of that, Jack realizes his father is just helping, and doesn't allow their relationship to become strained.
Jacob congratulates Sun and Jin at their wedding and tells them that their love is special and that they should not take that for granted. That bit of advice could have changed them and allowed Jin not to let his work to get in the way, and they both become completely honest with one another.
As far as John Locke, he may not have been initially dead from the fall, but as a result became paralyzed at the waist down. Jacob could have come and healed him completely, and Locke doesn't end up in a wheelchair.
These changes would completely alter who these people become and would ultimately prevent them from going to the Island, at least initially on Oceanic 815. Without those people on that flight, it would change everything that happens on the Island. As far as the visits with Hurley and Sayid, those take place after 815, and those happened to get both of them back on the Island. Just a thought, but in all honesty, I think they were just flashbacks. It was just something that got me thinking.