If "not planishing" was the reason, maybe could not get behind the weld with a dolly? Possibly could weld the outside repair first to be able to access back. I was also going to say that too tight of a gap/no gap can cause, but you say you used the blade style clamps which do leave a good gap.
We are not sure exactly what you have ended up with. May be fixable, may not. We would need several good, well lit, pics to see what you see. You may be describing warp and you may not. After we see pics, it may be possible to grind off weld and re slit and re do. If warped, you are screwed. I think you probably tried to go slow and tried to skip.
It seems like the remedy will be to grind off the backside welds, saving the piece to be reused. Then planish with reach-in dolly backer and see if "pucker" goes away. Depending on posting pics first.
Some say get as tight a gap as possible, some say leave a gap. The clamps you used generate about a .040 gap. This point is debatable. Some of the "tips" seen on the internet can conflict with each other.
Remember: Metal EXPANDS when heated. If gap is tight, it may "pucker". Metal SHRINKS when it cools. It may not, probably will not, flatten out the pucker. Even planishing may not. When the two "tectonic plates" collide, the mountains rise up at the joint or worse. So this is where the battle of the butt gap opinions begin. We are, of course, talking thin sheet metal. The correct gap is between zero and 1T.
Also, as Robert linked, it mentions NOT welding both ends of a panel first or NOT doing a perimeter first. This may trap the piece and not alow it to move. He seems to indicate to start at one side and move across. Did you do that? I see you clamped both ends first?
Many times I am unable to planish and have not had any bad things happen.
I see inner and outer door "bottoms". No skins. Can you post the vendor and part numbers for reference? Don't see an inner skin. I see there is an access cover inside bottom of door.
The inner turned out fine because it had some non flat features (ridges, raised/lowered ribs, dimples, changes in direction, etc). These will help eliminate warp. The outside is flat and smooth and long. Those welds are the most dangerous.
