Took me ages to write that post too, translating it from the 'PROCEDIMENTO CONSTITUCION PAREJAS DE HECHO' information sheet. I would also say that the process cannot completed before he & she arrive in Spain. Submitting the application at the relevant "oficina de registro & información" has to be done IN PERSON (presencial). When I first applied for my NIE in 1992, I had to provide evidence of an offer of work, i.e. a contract of employment, which my language school duly provided. When I renewed in 1997 the whole process took nearly six months. Now it's no longer necessary for EU citizens to apply for a NIE. You just apply for a 'Certificado de Ciudadanía de la UE' (Policia Nacional). This, in conjunction with your own national ID card and/or Passport, is your formal ID in Spain. The requisites will be different for those from outside the EU though. I'll be taking out Spanish nationality soon, so I'm preparing myself mentally and physically for a 16 round bout with the burocrats, lol.

My family and I flew to Madrid with one-way tickets because we weren't sure how we were coming back (we had plans to go somewhere else after 6 months in Spain. That was in 2004. We didn't need to show any proof. But we did have to go through secondary inspection at the airport.

#25 where are you from? Your saying you stayed 6 months suggests you either had an EU passport or long-stay visa, or you overstayed (illegally) for 3 months.

I would buy a fully-refundable one-way out of there (e.g. to Morocco) and show that if requested.
Try to disguise the fact that it's refundable, i.e. just have an itinerary with a booking number.
They do often ask for proof of onward travel these days, but nobody has ever really checked it - and it usually happens at the check-in counter. Still, I would not risk a fake one. A fully-refundable one costs, after all, nothing.
Good luck!