No need to worry about any damage and by looking at your photos everything looks fine. (from the bottom side - how it on top?)
if there was excessive amounts of water, i mean like massive pools of water, that could have caused the plaster board to bend on the weight, then it would need replacing.
In you case it was a minor leak, so if the leak is fixed:
- Leave the plaster to dry natuarlly over 1 or 2 weeks
- Clean the area with a rough material, slightly damp with water- do not use soap.
- Let the surface dry.
- Pre paint any discoloured patches with the colour you want, several times, letting it dry in-between coats.
- Paint the whole ceieling to match the colour with 1 or 2 coats.
Nothing else needs to be done.
Lightweight spackling compound is not suitable for exterior use. Even if painted, water will effect it greatly. In addition, it is prone to cracking as it is brittle and probably will not withstand opening and closing of the door (vibrations).
Wood filler or wood glue may work to fill the crack and would be better for exterior use. The glue is probably more flexible than the filler and may be better for filling the crack. I've used the wood filler before, but usually to fill nail holes or fill in small cracks. For a few bucks and some sanding, it may be worth a go round.
Another option would be to caulk the crack. Since caulk is flexible, it will resist cracking better than the wood filler. Both caulk and wood filler would be paintable. I don't think you can stain caulk, but I believe the door is painted on both sides.
The other option I mentioned in the comments is to replace the panel/door. The panel is most likely held in by surrounding trim, so if that can be removed, just the panel could be replaced.
Another option would be to get a very thin piece of veneer wood, cut it to the dimensions of the panel, and then glue it on top of the existing panel. Since you cannot see the crack from the outside, this could be done on the inside and then painted to match.
Best Answer
Knot stains happen when the sap of real wood transfers through the paint. Knots are full of sap, and don't fully dry out when the boards are processed. This can't happen with drywall.
What does commonly transfer through drywall is water, and I would guess that what you have here is a water stain. It could be a leaky pipe fitting, or it could be condensation from an A/C line if that happens to run through the wall, but I admit that's probably not very common.
Since the house is so new, just call the builder.