For all that it is, its not needed. Going back will only produce a larger wad of weld, and unless you really crank up the heat, you'll likely miss the heat settings of getting the "old wad" melted into the base and adding new weld to it.
This is essentially the biggest drawback to MIG: getting the heat right. You only get a single chance, else you are grinding it out. If you run a hot bead over a cold bead, the lack of fusion on the cold bead never gets fully repaired with subsequent passes (especially not subsequent tacks). TIG, you can wash the earlier beads with just the torch and fuse everything up, then add more and go. Its not ideal, but it does allow a bit of "repair on the fly" work for things like this.
Your work will hold up fine. A gentle filing down, and you're off-
Regarding the seat foam, the shape needn't be dead spot-on in its raw form. The upholsterer will still need to use a thin pad under the vinyl, and can finesse the joint to the tank. An electric carving knife is a perfect tool for shaping.