On the controllers shortages at SFO , I do not have the exact figures but Nationwide it is between 15 and 20% , mostly compensated by overtime . NATCA has an interesting piece on the US controller shortage on their web site :
http://www.natca.org/
The total number of fully certified professional controllers (CPCs) has dropped 10 percent since 2011 and continues to decline. The most recent FAA count indicates there are 10,532 CPCs. This number represents both a 28-year low and the disturbing continuation of a sharp five-year decline. The lack of a stable, predictable funding stream has made these problems worse. Despite some incremental progress since late 2015, the current total of CPCs is more than 2,300 short of the FAA’s overall operational target of 12,896.
But as to the question : Would an extra controller had spotted the deviation earlier ?
I frankly do not think it would have made any difference in this case , but that is only my opinion.