If your answer to that is "use a different spell," why bother with the nested Twin trick?
The answer varies quite a bit by circumstances. Some immunites like "immunity to magic" are dealt with directly. Others require indirect attacks, with the worst case being pretty bad.
Consider mind control. The standard approach to mind control is enchantment[charm] spells for which immunity is common at high levels. Instead, you can autohit with Necrotic Cyst by stripping the SR:Yes and Twinning through the Fort Neg followed by stripping the SR:Yes from Necrotic Domination and Twinning through the Will Neg. Or you could Disjunction followed by a Charm Monster that is SR:No with (effectively) no save. The first captures an enemy humanoid spellcaster intact in the middle of a battle. The second is an autowin against spell based defenses like mindblank + high saves.
I parse Wish(Metamagic(spell)) -> (spell) and Metamagic(wish(spell)) -> Metamagic(spell)
My understanding is that by RAW you can Wish(Metamagic(Spell)) since a metamagic spell is a spell. This is ridiculous, since you don't pay for the metamagic. My solution is as per the errata of Arcane Fusion. Yours is as above. Yours would weaken the approach substantially, while I expect mine would be more common.
You wanted to know the line between TO and CO and you're approaching it merely with this sentence.
That's good to know. I hadn't realized that approaching TO was as easy as full archivist spell access.
Well if you want to please others, offering your build in phases would be good. People tend to be a little aggressive here than you might be used to.
Let me suggest two lower power variants of this build, based on a Sorcerer and a Wizard.
Stutter Sorcerer
Human Sorcerer 4/Cleric 1 (Magic, Rune Domains)/Dweomer Keeper 10/Lore Master 5
Feats are:
1. Invisible Spell
(Human) Cooperative Spell
3. Arcane Preparation
(Domain) Scribe Scroll
6. Sanctum Spell
9. Twin Spell
12. Arcane Thesis[Arcane Fusion]
15. Arcane Thesis[Limited Wish]
(Frog God's Fane) K(something)
18. Arcane Thesis[Greater Arcane Fusion]
19 or 20. Arcane Thesis[Wish]
Stutter Wizard
Human Wizard 4/Cleric 1 (Magic, ?? Domains)/Dweomer Keeper 4/Wyrm Wizard 2/Dweomer Keeper 5/Wyrm Wizard 2/Dweomer Keeper 1/Lore Master 1
Feats are:
1. Invisible Spell
(Human) Cooperative Spell
(Wizard) Scribe Scroll
3. Sanctum Spell
6. Twin Spell
9. Versatile Spellcaster
12. Arcane Thesis[Arcane Fusion]
15. Arcane Thesis[Limited Wish]
18. Arcane Thesis[Greater Arcane Fusion]
(Frog God's Fane) K(something)
20. Arcane Thesis[Wish]
Both of these builds rely on neither Dragon nor Unearthed Arcana material and use no strange entry tricks. They both drop persistent spell, but retain the full ability to do recursive twinning. The end state for spell access is Sorcerer 19/Cleric 1 and Wizard 17/Cleric 1 with spell access lost gradually throughout the builds making them fairly playable throughout. In short, you can't complain about the _means_, as some of the earlier responses did. Are these considered TO in your opinion?
Allow me to explain where this hits TO.
That's helpful thanks.
If you optimize offense, then you are optimizing to defeat the generic defenses. Presumably degree of success is then the deciding factor between CO and TO. But the requisite degree of success is not clear to me. Let's consider Monster Manual opponents as a baseline.
(a) The Mailman build pretty much always hits, and can inflict an expected 285 damage at L20 with a single SR:No, no save, touch attack spell: GAF (Tw Ma Orb, Tw AF (Orb, Tw Ma Orb)). Given that this build can cast ~4 spells in a round, this is an autowin against basically all Monster Manual monsters. Is this TO? It seems like much of your discussion points in the same direction.
(b) The ability of Stutter to breeze through multiple encounters might be overestimated as there are only 4 Supernatural Spells/day which can generally only be used to overwhelm 1 or 2 opponents. Without the Su spells, Stutter is reduced to Twin GAF [L4, Twin AF [L4, L1] ] which is not so fierce. Using Orbs, this is 405 expected damage if you can make the touch attacks.