One bit to consider, since you've asked about elite miles, is that not all NW fares will generate 100% EQMs in the OnePass program. Flights booked in V/L/T/K will only earn 50% EQM credit and 0 EQPs. Otherwise you'll get all the same benefits, including EUA, RDMs, bonus RDMs, etc.

As far as the EUA process, CO elites and NW elites are intermingled for the upgrade process, so as a CO platinum you'll go before a NW gold, not after all the NW elites.