4. That would be more reasonable. But it would also be fine to have DR be active all the time, and follow Barbarian DR progression to help fill dead levels.

6. The paladin can summon his mount 1/day, whereas the Ghost Rider has no such limits. So even if it only lasts 2 hours/level, after it expires, the Ghost Rider can just resummon it.

Although, I think this is actually fine. Given the modifications made to the Mount, I would say it would now be reasonable to make it summonable/dismissable at will, and remain as long as the rider wants. This would at least focus the Ghost Rider into a mounted combat warrior, and place it on par with the Barbarian/Rogue (tier 4), or even Warblades/Crusaders (tier 3) depending on other class features.

In that case you might want to change: "Each time the mount is called, it appears in full health, regardless of any damage it may have taken previously."

Into something like: "For each minute where it remains unsummoned, the Mount recovers 1 HP/ Hit Dice. If it remains unsummoned for at least 10 minutes, it is cured of all other negative effects as well (e.g. disease, energy drain, curses etc)."

Otherwise to heal his horsie, the Ghost Rider simply has to unsummon and resummon immediately, and it will reappear at full health.

11. In that case, I would not use "halved" in the wording at all, since most Fort/Will save targetting attacks do not suffer half effect on successful save. I would phrase it as "At 9th level, the Ghost Rider still suffers no effect on a successful saving throw. Additionally on a failed saving throw, he now only suffers the reduced effect (if any), as if he had succeeded on the saving throw without having Mettle."

Incidentally, this is a tremendously powerful ability. It is in fact almost as powerful than the undead type. It won't work against no-save ability damage/drain and energy drain, but at the same time it protects against things even undeath does not. (E.g. Undead can still be disintegrated, or transformed into glass by Glass Strike, whereas Improved Mettle as written effectively reduces/blocks all of these).

Given its power, I would make it the capstone either alongside, or in place of gaining the undead type.

Or place a scaling limit of some sort (e.g. if he fails the save by within X points, he only suffers reduced effect, and then as he levels up, X increases. And at the Capstone, X limit is removed).

13. The ability to take a move action as a swift is not over-powerful. For example, the Psychic Warrior has hustle, so that he can do it until his PP runs out. So you could start with Malign Fury 1/day, and then slowly scale it up as he levels to fill out dead levels.

On the related topic of filling out dead levels, you could make Hellfire Blast start smaller (than 1d8), and then scale gradually as well. E.g. Start at 1d6 at level 3-5, and increase by 1d6 every 3 levels.