A) Nerull cannot cast 9th level arcane spells. He casts up to 5th. So Reaving dispell is out.
He can cast Greater dispell magic as divine spell, that does cap to 15 so Nerull is infact unable to dispell Jimmy's deathward.
Alter Reality; duplicate 9th level spell or lower as a Standard Action. Assuming RAW Psionics Transparency, applies there as well.
C) Nerull doesn't have selective spell. Or extrodinary spell aim or any other way to exclude himself from AMF. If he uses Miracle to cast AMF with a metamagic he doesn't have (I didn't even know it could be done). Jimmy would respond with Celerity+Planeshift to Celestia and be free and alive.
Again, Alter Reality; "duplicate a spell with any metamagic feat (so long as the metamagic feat is available to characters of 20th level or lower"; Selective Spell certain falls within that area.
Jimmy would be plain shifted to Celestia. All that Nerull would have to do would be to get one of his mortal followers (perhaps another Cleric 20 gated in), and use Miracle to PsyRef in Astral Tracking, and to give them the required Survival skill ranks to bring him home.
Celerity abuse isn't going to happen; he has 6 4th level Arcane spells a day; which he can use his Spontaneous Wizard spells to use all of them for.
Lets also have a look at his Salients;
Alter Reality; already explained. Wish on 'roids. One thing that's important is that he can render magical/supernatural affects permanent. With enough preparation (he of course knows about it), he can every 60 hours permanence and effect.
Alter Size; irrelevant at this level of optimizing.
Arcane Mastery; no need for a spellbook. And it can invent new spells without research. There are so many shenanigans involving an ultraintelligent individual like Nerull being capable of coming up with something.
Divine Shield/Area Divine Shield; Basically add 1700 to its hit points total.
Annihilating Strike; so anytime your Cleric is hit by Nerull, it must pass a DC60 save, or be reduced to -10 HP. Unless you're pulling off Persistomancy delay death+beastlands ferocity, considering that Nerull is making 5+ attacks a turn, some will hit.
Avatar; presumably irrelevant
Control Creatures (undead); Not relevant until they're Gated in. Or until after your cleric has been made a plaything.
Craft Artifact; He probably has some sort of Artifacts knocking about that he could use.
Create Greater Object/Create Object; Irrelevant.
Divine Blast; again, 26d12 (average 169) 12/day, no save. Again, immunity to HP damage required.
Divine Celerity; hasted is always good.
Divine Creation; this is better now, Full Round Action to create mortal creatures. Create himself a Prismatic/Force/Time Dragon shall he? Sure, he has to rest, but so what?
Divine Spell Focus; +17DC to Necromancy Spells, he can create his own spells without research, QED, any spell he has is a Necromancy school version of it.
Increased Spell Resistance; SR sucks.
Hand of Death; provided it's immunity hasn't been dispelled, immune. Oh well. If it has; DC48 or death.
Life and Death; see above.
Life Drain; Can you see in the Dark? This isn't a death effect either; DC38 is a bit more manageable though.
Spontaneous Wizard Spells; Celerity Wars? Game on.
Supreme Initiative; already addressed.
Undead Qualities; does exactly what it says on the tin.
So no, I'm not sure what you're trying to prove. That an optimized character going up against a God is going to end in a severely smushed character, even ones as "poorly" optimized as the per book deities, because said god has access to countering immunities. Or that immunities are bad because you're not capable of coming up with enough ways to bypass them?
Even in low-OP, Searing Spell allows your Fireball to hurt Red Dragons.