Protean Scourge trick is cool, and I hate to poke holes in such a cool trick, so I'll try and patch them too.
//Oh wow. Didn't expect to find that.//
I'm pretty sure you need to polymorph (or similar) into the protean scourge's natural form for Assume Supernatural ability to work. This is easy to solve, just take another assume supernatural ability to get their shapeshifting to look normal (or be okay with looking different).
Whether changeling minor change shape would count as polymorph like is pointless, because changelings aren't tall enough. Changelings are at most 5'9" on the random height table, and disguise self is limited to + 1 foot of height. Protean Scourges are 7 ft. tall. This is unfairly early access, and other magic should work, so it's not a big loss. So you'd need an abnormally tall changeling, and why go through the hassle when their are more reliable ways?
Sadly, Humanoid form invocation doesn't work either. Protean Scourge's are Monstrous humanoids. Because I'm not hugely interested in digging through hundred of other options, I'll jump to the most reliable one.
Polymorph. Pretty much the example spell for Assume supernatural Ability, can clearly get us into the form of a protean scourge, and otherwise does everything we want. Even better, it fits on a wand, so buy a single charge wand on our factotum or warlock and win via umd.
But jojolagger, it only lasts a minute per level, do you really expect us to generate enough clones in just a few minutes? Honestly, 13 minutes (lowest CL this works with) of clones should be easily enough. Assuming a split every 1/2 minute, that's 2^26 clones, 67 million. You now out populate the five nations of eberron combined. But we don't need to stop there. Because we don't change back. You see, while split, neither half can change shape. Polymorph turning off would change our shape.
Also, spellfire levels stored with spellfire wielder shouldn't be shared like spells, so we should grab it for even more stacking power.