So, let me get this:
XYZ cannot be Monster Reborned, since it was a Fusion Monster not properly Fusion Summoned.
No, I didn't say it couldn't be Monster Reborned, I said it could because it's a fusion monster that is not "fused" it is simply special summoned. It simply reads "You can only special summon this monster by removing..." and doesn't read "This card can only be Special Summoned by Fusion summon." like Dark Paladin reads, for more on this look down.
XYZ can be Dimension Fusioned, since its Special Summon conditions have been completely met.
As long as it's first special summon requirements were met it can be special summoned from anywhere again (other then the deck directly in case of a Fiber Jar). As I said before it falls into the category of BLS, CED, and Necrofear in effects like this.
Fusion Monsters brought by Metamorphosis cannot be Monster Reborned, since they were Fusion Monsters not properly Fusion Summoned.
This is correct because to special summon a normal fusion monster from the graveyard you must have fusion summoned it to begin with. Since a fusion was not properly fusion summoned, it can not be monster reborned. Don't look at XYZ as a fusion monster because it really isn't, the only thing that links it to a fusion monster is the fact that it's classified as one and goes in the fusion deck. It is never fusion summoned at all, it's only special summoned by removing the pieces that make it from the game. So this means you don't need Poly or Fusion Gate, which are the only ways to fuse an actual fusion monster.
Fusion Monsters brought by Metamorphosis cannot be Dimension Fusioned since its Special Summon conditions haven't been met.
Yes, look above to Monster Reborn for more on this. Like I said, XYZ is never fusion summoned, it's simply special summoned through it's effect. I'll quote a ruling on XYZ for you. ""XYZ-Dragon Cannon" may not be Special Summoned by Fusion Summon." This means that it isn't fusion summoned, it's simply special summoned by doing it's requirement.
Dark Paladin cannot be Monster Reborned or Dimension Fusioned, since it is a 'nomi' monster.
Correct, because it is a "nomi" monster. Dark Paladin reads that the only way to special summon it is by the fusion summon and it's the only way it can be special summoned, you can't special summon it any other way. I know things get confusing, but if you read the wording on the cards, it's quite simple to follow.