Multiple wh-fronting in Spanish?

I discovered, to my surprise, that multiple wh-fronting is sometimes possible in Spanish:

(1) ¿Cómo, por qué, para qué, a quién enseñamos Español?

This literally means: "How, why, to what, to whom do (we) teach Spanish?"

Furthermore, the order of wh-phrases is actually free in these cases. However, multiple fronting is blocked if the subject is questioned:

(2) * Quién cómo compró?

Direct Object questions also seems to have a blocking effect:

(3) *¿Qué cómo compraste?

but they can be ameliorated with a resumptive clitic and coordination (to my judgement, anyway):

(4) ¿(Dime) qué y cómo lo compraste?

And DO-IO questions are not bad (I think):

(5) Qué (y) a quién (le) compraste?

So, when we question non arguments, we can freely front wh-phrases, but with arguments it seems to be some restrictions. I was thinking about that. Do you know if this has been noticed (not only for Spanish, but other languages)?