Let me put this bluntly...
When people are talking about the popular vote in conjunction with calling this past election illegitimate because of it, the idea they are "sore losers" becomes quite obvious. When they talk of changing these rules using this election as the example, I'm not sure how else one can fairly characterize it. When it comes to the numbers, I think I just explained why just adding all states together doesn't give reasonably accurate results. The campaigns would have campaigned differently. Voters in solid blue and red states probably would have voted in greater numbers. No idea who would have really won a popular vote but adding each state together doesn't prove a thing.

As to changing the rules, they can try. I think it takes 2/3 of all states to vote for the same plan to change the rules or a 3/4 majority because this plan is in our Constitution. Good luck trying. I think I remember reading that this issue of popular vote has been discussed 700 times in the last 200 years and they could not come to an acceptable compromise.

Addressing your other points. Heck, when I was in line to vote, there was a woman ahead of me holding two ballots. Apparently she messed up her first one and the poll workers gave her a replacement without taking her other one first (maybe so she could transfer her selections? I don't know. They almost didn't take the second ballot from her. She could have put it through the machine twice. It seemed to be handled correctly eventually, but this stuff happens. It's why there are recounts. Votes are sometimes miscounted. A poll worker writes 67 being a little dyslexic instead of 76. People vote on Provisional ballots but they had forgotten to register, or had moved and forgot to change their residence so could have voted twice in two different districts. Shit happens and every time there is a recount the vote pretty much always changes a little. Ask Al Frankin. And I don't want to get into the issues with hanging chads and butterfly ballots...

There are further complications. The USA is a republic of states. Each state is empowered to create their own rules for election. It's called states rights. Most states are going to object to losing the right to elect by their own standards. Making one Federal law on how an election needs to work...good luck getting enough states to agree. Same 2/3 3/4 to make such a change.

As to swing states. The country changes and every election is different. In 1964, a democrat won all but 6 states. In 1980, a Republican won all but 6 states including New York and California. What becomes a swing state in any given election cycle develops over each 4 years. There is no guarantee the same states will be the swing states next time depending on the candidates.

Besides, "swing states" is a media creation anyway. You are right, every election has a handful of swing states. Which states are they? Find all the states which are close between two candidates, add up the ones on either side, decide winning which states will probably win the election. Those are the swing states. They don't start as swing states, they become that after the election has gone on for a bit and the popularity of the two candidates and where solidifies. Even then, polls are often wrong. Wisconsin and Michigan weren't close enough for Trump to win according to the polls...there is a reason people actually vote and we don't rely on phone calls to a couple 1000 people at random...