Greetings scalpel,
The etymological background for çünkü is not important for the learner. The point is that 'çünkü' has no suffix in Turkish. There is no word such as 'çün'. Maybe in Persian there is. I don't know and I'm not interested.

Let's take these examples:

dün (yesterday) > dünkü (the one yesterday)
burada (here) > buradaki (the one here)

Çün (non existent)
çünkü (because)

I agree with you about the etymological background of çünkü. But these are not important for the Turkish learners and they only cause confusion.

I see your point. When putting it that way it makes more sense.

In short, I think, looking for a smillarity between 'çünkü' and any word that has a relative pronoun (dünkü, buradaki etc) was a bad idea but pointing to the fact that 'için' and 'çünkü' have a relation between each other that comes from their etymological backgrounds, is a good point. To tell the turth, I didn't know these words were etymologicaly connected.

I think your last examples are a good educational approach to the issue.