Good post. I think you got almost all of it. One thing that one should add is that the response rate to any poll has declined over the years and is now (according to PEW) below 10%. (NB. Very rarely do the polling companies tell you how many interviews they have started, or how many phone numbers they have called).
As I have written elsewhere some research articles indicate that the errors in the polling have not increased despite the reduced response rate. That may well be true, but one has to be careful. Any election where there is more reason for voters for one candidate to not divulge their choice can cause very large discrepancies in the polling results.
Given the media’s relentless pursuit of Trump we may (cannot prove it, but we may) have such an election now. In that case the polls are not worth anything.
I agree, and one thing people might forget is that IF a large swath of Union voters are going to vote Trump, they may be the most unlikely of all to answer a poll, and we all know the reasons why they might think that way.