Now this assumes I can retain the same level of employment between ages of 50 and 54. It also assumes that future expected returns are not impacted by the pullback. I'm still wrapping my head around that one. I believe the market is essentially a giant discounted cash flow mechanism so that if we get a pullback that should mean returns on the morning of that pullback should be higher over my expected lifetime then at the peak of that pullback. I assume 5% nominal return for equities in my calculations. If we had a massive pullback like that presumably I could use a higher expected rate of return going forward which would reduce the 4 years calculated above??

I think your assumptions are all correct. 5% nominal returns means that over 30+ years average returns will converge to 5%. If it is lower now it should be higher in the future, otherwise it will not average to 5%! The exact date you retire will absolutely affect your initial rate of return, which will dictate the age at which you can retire with 100% success rate (which is what is sounds like you are trying for).

You already have a conservative asset allocation and a conservative SWR. As you have already concluded, if you are any more conservative you are going to guarantee that you have to work longer.

In hindsight, if you had $750k in socks in 2009, you could have retired on $40k/year. That is not the case now. But how to explain why that's the case?

$40K/year w/ $750K was OK something like 65% of start years... how do you know that it is not the case now?? Also, please tell me your socks are only mismatched by color and not length, otherwise your whole sanity is in question ;P