I've never really had a problem separating Race from Culture since way back to my first MERP days. If an elf was raised by Rohirrim, the character had the elf stats and the rohan adolescant skill development. Pretty simple. The rest is just an application of common knowledge.
The only real sticky point that made us struggle with was the Background Options. It seemed like BG pts were partly due to culture, but also partly due to game balance. Most races that had a lot of stat bonuses had fewer BG pts to spend, so in the case of an elf raised as a man (2bg vs 5bg) we usually split it based on what was gained/lost in the culture switch. For example, this elf would have lost a lot of starting languages. We figure that's worth a BG pt. But we figured the +20OB on Horseback was more inherit to the Rohan race then their culture, but we offered it as a BG option (i.e. 1BG pt to get the bonus) if you were raised in the Rohirrim culture.
Not sure what else is really needed from an "official" standpoint...
-shnar