These are all good questions with uncertain answers from my perspective.  There is no one distinct area that is important.  Any wind anomaly over the equatorial (5S-5N) Pacific is something to watch and monitor because of its potential influence on surface and/or subsurface temperatures.   In general, wind anomalies that are persistent or strong (i.e. for El Nino, not just westerly winds in the *anomaly* but also in the *total winds*) have more of an impact than more fleeting, weaker anomalies.  

As for the impact of the most recent westerlies, I believe it has already helped to keep some of the surface warmth in place (for example, we saw an abatement in the decrease of the Nino-3.4 index over the past week).  Whether it helped to force a new oceanic Kelvin wave, that still needs to play out a bit more in the observations, but there is a chance it did.  

I find these indices of subsurface ocean temperatures to be useful.  They are based on data from the NCEP Global Ocean Data Assimilation System (GODAS):

 http://www.cpc.ncep.noaa.gov/products/analysis_monitoring/ocean/index/h…

More GODAS information is available here:  http://www.cpc.ncep.noaa.gov/products/GODAS/