Con did a great repair, and made a very nice document to illustrate it.
But I also think you could get good results by reshaping the door (as you suggest), and re-fiberglassing the inside. Now I have to qualify that, because I'm not sure how your B-17 door is made, and even all B-13 doors I have seen are not made exactly the same. So this goes for a door that is made up of two fiberglass skins with some coring in between them.
In this type of construction the skins are in tension (or should be), and so the unit of skin-bonded to-core-bonded to-skin functions a bit like an I-beam. If the skins are de-bonded - either from water getting in and rotting the core, or from a problem during construction that left voids - then the ability for the "unit" to keep its curved shape is diminished.
If the door seems dry inside and you don't think there are large voids or de-bonded areas, then I think you could do as you suggest with the kerfs, re-bending, and re-fiberglassing. Another idea would be to cut off much of the inside skin (leaving some around the sides to bond to would make it easier to put the skin back on), and then either remove and replace the core (if it's wet or unsalvageable), or just put on a new skin.
One you have good skin on both sides, that's well bonded to good, dry core material, I think the door should be able to hold its shape (although I more typically work on boat decks; they use the same principles).
Other people have chosen the more simple route of addressing the symptoms, and put on various "inside webs" in the form of angle irons and wire-rope with turnbuckles, or wooden shelf units, etc. that make brackets to hold the shape.
Another thing to check is that the trailer sides around the door are not sagging (bulging in the middle). On B-13s this happens more on the hinge side of the door, which takes more stress and also has less support. On the 13s there is a piece of bar stock there, but that is not always effective. A nice solution I've seen is to put in a tall "half moon" shaped piece of plywood that is perpendicular to the door, forming a "mini wall" that sort of triangulates the trailer there and keeps it from bowing out. In other words, this is a floor-to-ceiling piece of, say, 5/8" ply, that is like a mini bulkhead running crosswise in the trailer, attached right at the door frame.
Also, if you have the cast hinges with the brass balls (like the 13s), then the hinge can sag when the ball wears down; these are rebuildable with kits from Scamp