Now let the low branch that will be the new trunk grow wild for a couple years. You want it to become at least 1/2 to 2/3rds the diameter of the trunk below. This will give you believable taper, a logical transition from one trunk segment to the next. By letting it run wild, I mean let it grow. If it grows to 6 feet tall, don't worry, that is exactly what you want. Once or twice a summer measure the diameter of the main trunk and the diameter of the low branch and when it is in the right size range, you are ready for the next step.
The back branch can go at any time. You could leave it this year if you want, but certainly next spring get rid of the back branch. Or if you are set on keeping it don't let it extend much. Let it grow maybe 6 leaves worth of extension, then chop it back to just one new leaf. You will have to repeat prune several times over the summer to keep it subordinate to the new trunk. If you keep it eventually after the tree has had a chance to heal, maybe 2 years from now you will carve the old trunk stub to look like it was hit by lightning or wind or some other disaster. My personal inclination today would be to take it off entirely, but that is my personal taste, you may want to keep it and I think it is possible to incorporate it into the design. You can always remove it later.