2ManyTrucks is correct in that the metal shrinks lengthwise along the weld, but you should be able to minimize that as you planish the weld dots. You may have to do some shrinking, but as he said, figuring out where and how much to shrink is a bit difficult. Oil-canning is indicative of a panel that's been stretched (or shrunk along the edges - the effect is the same).
Your latest attempt looks pretty good so far. I would complete the welding of the patch and deal with minor bulging after you're done.
