Am 18.02.2019 um 15:53 schrieb SuStel:

Imagine you instead asked, /Why do I need to feed your cat if I help you?/

    *qaQaHchugh, qatlh vIghro'lIj vIje'nIS?
    qatlh vIghro'lIj vIje'nIS, qaQaHchugh?*

You have just interchanged the chugh-part, which is not allowed with meH-clauses.

Intentionally, to more clearly illustrate the boundaries between clauses. The main clause remains the same, and if we can move the if-clause we can see where the boundaries of the main clause are.

There is at least one canon example of placing {qatlh} in the middle of a sentence. But isn't there a difference between a {-chugh} sentence and a purpose clause? (I'm seriously asking)

A difference, yes, but does the difference put the purpose clause inside the main clause, while other subordinate clauses remain outside of the main clause? I'm not convinced of that.

How about this: jagh DajeymeH nIteb yISuvrup In order to defeat the enemy, be ready to fight alone. Adverbials come at the beginning, barring time expressions. Yet the purpose clause comes first in this sentence. qatlh is basically just a question form of an adverbial. Why isn't it nIteb jagh DajeymeH yISuvrup?

But I can also supply a counterargument. vaj malopmeH tlhIHvaD nob SaSuqpu' So to celebrate, I've gotten you all a gift. (Hallmark commercial) Here, the adverbial vaj is placed before the purpose clause.

So I don't think there's an exact answer here.

-- 
SuStel
http://trimboli.name