To answer the actual question, which was "would you teach lefty to start over again?" I would. I did. Some 12 years ago I had a little girl who didn't know right from left, nor baseball from softball from volleyball. She did have a baseball-minded father who was a lefty/lefty. I knew that she would be 2 steps closer to 1st if she was lefty vs righty, regardless if she was fast or slow, and lets be honest here, If we are talking about "starting over", at that age, who knows if she will be fast or slow as she gets older. Anyway, she was all right-hand dominant, but she batted left-handed. It's all she ever knew. She's now a Junior in HS and as it turned out, she runs like a tank. LOL She has had hits through the years that were bang-bang and I'm certain she's out if she's batting from the right side.
I agree with some and disagree with some on the slapping aspect. If your lefty wants to give it the time, TEACH HER TO MOVE IN THE BOX!!! It's not always about being fast, but with the ability to slap or drag, YOU can place the defense into a position to your advantage. My DD learned to move in the box at 12 and she really pulled her front side off the ball that year, so when moving, she seemed to actually keep her front side in and she hit the ball more solidly. She did have issues with strikeouts one year, so we moved her back right to see if it was a dominant eye issue.....It wasn't. One swing and we knew, she was not a righty batter anymore lol.
She doesn't move in the box much, but she has the base there that she can if asked. In HS games she has drug some really nice bunts the past couple years in situations where the defense never saw it coming. She has moved more in winter leagues this year more than ever. I think she will move more now just to confuse or "set-up" the defense that she wants.
My second DD was 2 years younger, and I did the same thing. When she was 5 she asked if she could bat right-handed once. I said sure. She hit it to the fence. Lucky swing I thought. Next swing, same result. I told her she could swing from wherever she wanted. She has never seen the Left side of the plate since. These 2 "sprinters" are now hitting 5/6 in the order together and just burning up the base-paths. LOL
In all the times I have turned a girl from right to left I did do 1 thing. I taught them to bunt, drag, slap, and hit. They also were never given the option to go back to righty. I thought that if struggles arose and given the option, they would always revert back to what was comfortable.