How does Quicken Spell-Like Ability work with shape/essence modifiers?

Question

Warlocks can take quicken spell-like ability: eldritch blast (MM p.304) at 10th level (really 12th level due to feats from level progression). And from everything I've read, modifiers on eldritch blasts do not change the fact that it is an eldritch blast so you don't have to take the ability for different combinations of eldritch invocations applied to that eldritch blast.

However, there is a potential issue when reading the description for Quicken Spell-Like Ability in the monster manual:

The creature can only select a spell-like ability duplicating a spell with a level less than or equal to half its caster level (round down) –4.

The issue is this: When selecting blast shape and essence invocations, they affect the level of the spell-like ability. So while you can learn quicken spell-like ability for eldritch essence (because it is level 1), casting an Utterdark Blast is a level 8 spell-like ability. It is unclear if a creature is allowed to actually quicken that application of the eldritch blast, or if they can only quicken applications of eldritch blast that still conform to limitations set by the caster level chart on page 304.

Answer 1

It’s unclear, unfortunately, whether the “selection” is a thing done once, when you take Quicken Spell-like Ability, or if you must constantly re-select the same choice every time you want to use it—that is, whether or not you have to re-check the spell-like ability’s spell level each time.

If you don’t, then as long as you had CL 10th when you selected eldritch blast (which is necessary to meet the feat requirements anyway), you could select eldritch blast as the spell-like ability in question, and then no matter what changes happen later, you could use quicken eldritch blast.

On the other hand, if the selection is something you have to do again each time you use Quicken Spell-like Ability, then you have an issue. A brimstone blast may still “be” eldritch blast for the purposes of being the spell-like ability you chose originally, but brimstone blast does change the spell level of that spell-like ability to 3rd. Until CL 14th, Quicken Spell-like Ability would not allow you to select a 3rd-level spell-like ability. Therefore, you would have to wait until CL 14th to use it on a brimstone blast.

I feel that the rules text favors the former interpretation, of selecting just once when the feat is taken, but the rules text was also definitely not written with consideration for spell-like abilities that change spell level. That’s not something that spell-like abilities do—except for eldritch blast.

Answer 2

As you accepted SPavel's answer on your other question, I'd say using that logic, yes, you're quickening Eldritch Blast, a level 1 spell-like ability, the invocations and shapes are simply add-ons.

However, I still maintain that invocations are separate spell-like abilities and should be treated as such, in which case, no, you could not cast Utterdark Blast until epic levels.

From my reading of Brimstone Blast, forexample, it changes Eldritch Blast into Brimstone Blast, so would require a separate Quicken Spell-like Ability feat.

This eldritch essence invocation allows you to change your eldritch blast into a brimstone blast.{...}

-Complete Arcane page 132