What if there is a substantial difference between polling results and actual outcomes? There will be a strong temptation to make accusations of cheating or fraud. There may be some ways to rule out cheating as an explanation for divergence between polling and actual votes. First, let's identify the theoretically possible reasons why polling and actual votes may diverge.
How can you make an exhausitive list without including the largest reasons for divergence? Poorly constructed polls, lack of rigor in selection criteria/methodology and/or inability to randomly sample. Even legitimate divergences in whether you attempt to poll "likely" voters or all potential voters can be decisive.
Voting officials cheat in some way with ballots cast to give more votes to Trump or Clinton
All but the last of these should vary significantly based on State. No matter how likely or unlikely we believe it is that voting officials will cheat in the ballot count, I think we can all agree that it is even less likely that Republicans would cheat to help Clinton or Democrats would cheat to help Trump.
Of course the highest risk for this kind of cheating is in districts in swing or battleground states, or states with a large urban vs non-urban party disparity where one party is so dominant that the other can't even bring in local observers but has to bring them in from outside the community. It's no accident that Chicago and Philadelphia have had voter fraud issues and are the target for such complaints.
Illegitimate votes cast by non-citizens or multiple votes cast by the same individuals are accepted and counted
Not sure why you'd think illegal alien voting would cause a disparity between polls and voting? Unless you think illegal aliens would identify themselves routinely to pollsters as illegal? The concern with illegal voting is that it influences an election inappropriately not that it doesn't match the polling.
Multiple votes by a single individual, would on the other hand, if accomplished at scale and in a single direction potentially have an impact.
The second potential gap between polling and actual results (of fraudulently cast votes) has been of strong concern to Republicans. Voting laws have changed in part to address this concern, and even though some have been thrown out by the court, there remain significant differences by state in ease of access at the polling place. If there are gaps between polls and actual results because of fraudulent votes, it is logical to assume that this would occur most in Democratically controlled states with the least stringent voter ID laws and least in Republican-controlled states that have added voter ID laws.
Voting laws were not changed to confront the point of differences between polling and voting results, nor would such differences necessarily or even likely identify a problem with illegal aliens voting. This claim doesn't really have anything to do with your point, so I won't argue it further.
Reductions in polling sites and other barriers to voting reduce the turn-out for those most affected
The third potential gap (impediments to voting) has been of strong concern to Democrats, and so if this were to occur, it would be seen by Democrats under-performing relative to polls most in Republican-led states that have reduced access to polling or increased the stringency of voter ID laws.
This is the Democratic fake issue they use to argue against Republican concerns about voter fraud. In all honesty, no matter how many polling places have been reduced the coverage map is still completely adequate to allow everyone to vote on election day, and when you add in the ever increasing emphasis on early voting and mail in voting, the real access to voting has done nothing but increase.
The difference in the efforts by the two candidates in terms of investment/staffing for get-out-the-vote efforts delivers a higher share of potential voters to the polls for the candidate who had the biggest ground game
The fourth potential gap (differential GOTV efforts) would be seen most prominently in states where the difference between Clinton and Trump (number of campaign offices, number of paid staff) has the biggest gap relative to previous elections.
This is legit and can definitely make a difference where it converts a voter who wasn't likely to vote into one that does.
I'd add this is one area that the Dems have had a large advantage, they have a much easier time identifying clumps of likely voters that they can drive to the polls than Republicans do.
External entities somehow hack the voting machines to favor one candidate or the other
The fifth potential gap (external hackers) should vary widely over states, because there are so many different voting systems. If there is a gap between polls and actual votes due to hacking, we might see a tendency for states with similar voting technologies would be similarly affected (although we might only see swing states with similar technologies to be hacked).
You are completely wrong in how you'd trace the impact, you'd be highly unlikely to see similar impacts based on voting machines in different areas (unless you posit absolutely incompetent hacking). All hacking based manipulation can be expected to be concentrated in key areas where there is a good potential to hide the impact. If you found the same manipulations in districts where hacking would be completely useless that would in fact be shocking.
People hesitant to tell pollsters that they are Trump or Clinton voters
The final potential gap (people not telling pollsters their true favored candidate) should not be correlated by state. Republicans sometimes assert that support for Trump is not adequately reflected in polls because he is not seen to be politically correct; Democrats sometimes assert that there is a significant population of conservative women who are keeping their support of Clinton to themselves. If there is a nationwide trend of actual vote counts diverging from polling, then it would most support one of these hypotheses for voters not revealing their true intentions to pollsters.
Except this one is in fact an observed phenomena, for whatever reason, some people do misrepresent their position to pollsters. I don't think its hesitancy so much as it wanted to present themselves publically as on the "correct" team even if privately they disagree and will vote otherwise. I certainly know some women who absolutely will not say they are voting for Trump publically, because they have other friends who'd consider them a gender traitor. Have no idea what they'd tell a pollster.
Assuming that there is a gap between polling and actual results, we could use this analysis to identify which potential causes for that gap are the most plausible. I have tried to analyze this in a way that is fair to all sides; can anyone think of something major that I have missed?
If there is a gap, which there almost has to be given some of the margins for error (heck some polls are not within each others' margin of error), then you'll most likely be able to glean why only by looking at the individual poll mechanics. The most likely cause will be sampling error.
Voter fraud and manipulation may, but is unlikely to, show up in such an analysis. End of day, fraud can be effective and still be within the margin of error for a poll.