Undercasting and Magical Lineage

[FedoraFerret]

So, while working out the math on the benefits of casting Mind Thrust II versus Undercasting for Mind Thrust I and Intensifying (for the record, starting at level 7 undercasting and intensifying is statistically better), I realized there's three potential possibilities with how magical lineage interacts with undercasting.

1: All of the spells in the line are theoretically considered the same spell, just able to be cast in different slots. This would mean Magical Lineage (Mind Thrust) applies to all of them, which is overpowered as hell, especially with the two-trait combo.
2: When undercasting a spell, it's considered the original spell even though it uses the damage and spell slot of the lower level (based on the wording of the Undercasting entry). For instance, when I undercast Mind Thrust V as Mind Thrust III, it's still considered Mind Thrust V. This would mean that Magical Lineage would be useless on the lower levels of the spell as soon as you replaced them, but would have the same implication as option 1 as soon as you hit the higher level.
3: This is all ridiculous, each spell level is considered a different spell and I can only apply Magical Lineage to one, but it applies whether I'm casting the original or undercasting a higher level.

I'm like 90% sure it's number three because either of the other two cases is kind of ridiculous, but I just want to make sure.

[LeesusFreak]

Greater Spell Focus and other abilities also will key off this. Dotting for interest.

[Helix7901]

Dot

[Archaeik]

I'm quite comfortable saying it's #3, they're all different spells (albeit with a special rule about belong to the same family).

Undercasting Spells wrote:

Some psychic spells can be undercast. This means that the spellcaster can cast the spell at the level that he knows, or as any lower-level version of that spell, using the appropriate spell slot. When a spellcaster undercasts a spell, it is treated exactly like the lower-level version, including when determining its effect, saving throw, and other variables.