Can mystic theurge be entered early using Magical Lineage?

Question

Look at the trait Magical Lineage. Say at level 1 the character took a levels in either wizard or cleric. For the trait Magical Lineage, the character picks a spell that appears on both the cleric and wizard spell lists, like summon monster I. (The trait Magical Lineage is exclusive to a single spell, not to a specific class.)

At level 1 the character also takes the feat Heighten Spell. At levels 2 and 3 the character takes either a wizard level then a cleric level or vice versa. And, by level 3, the character has 3 skill ranks in both Knowledge (arcana) and Knowledge (religion).

So at level 3 by virtue of the feat Heighten Spell and trait Magical Lineage the character's able to cast a 2nd-level summon monster I spell as both an arcane spell and a divine spell. This lets the character take at character level 4 level 1 of the prestige class mystic theurge.

Does this work this way?

Answer 1

That... doesn't really work like that.

Heighten Spell is unique in which it actually changes the effective spell level:

Unlike other metamagic feats, Heighten Spell actually increases the effective level of the spell that it modifies.

It doesn't add levels to a spell, it changes the spell level. That's subtly different.

A Silent Fireball is a 3rd level spell, even if it uses a 4th level slot. That's a +1 level increase.

A 4th Level Fireball is a 4th level spell. That's a +0 level increase.

Magical Lineage can't reduce the effective spell level, just the extra levels added by metamagic feats. Since it can't enable you to cast a 3rd level spell as a second level spell, it would make sense that heighten spell would not work with magical lineage.

Answer 2

Two things to note here:

First, you cannot use magical lineage to bring a spell slot bellow the minimum level required to cast the spell, as can be seen on this FAQ entry. So for a 2nd level spell, that would be a 2nd level slot, for a 3rd level slot, that would be a 3rd level slot.

That said, the FAQ clarifying Heighten Spell also explains that a 2nd level heightened spell is a 2nd level spell now, which means you need a minimum of 2nd level spell slot to cast the spell. Since magical lineage cannot reduce the minimum spell slot required to cast the spell, it does nothing when mixed with Heighten Spell.

Jason Buhlman (Paizo's Lead Designer) has clarified this officially:

Magical Lineage was never intended as a way for you to actually lower a spell's level. It was put in to allow you to reduce the increase from a metamagic feat. So, no unlimited magic missiles. I will see to it that the language of this ability is clarified soon and I will get this added to the FAQ.