Yeah, the system isn't working for the future. Yang pointed that out. If his solution is the only one or even a working one is up for debate, but quite frankly the debate how to reform the system should start by now.

Does anyone know what the polling numbers were for Trump/Clinton in 2016 at this time? As well as for Obama/Romney in 2012 when Obama was up for re-election?

I've been monitoring this on a spreadsheet. I will update for August shortly, which will impact it I think a good amount.

RCP average January - July of 2016 shows Hillary leading Trump by 4.33 points, on average. 2020 January - July shows Biden leading Trump by 6.44 on average. So through July, Joe is doing better. But the REALLY interesting part is that enthusiasm polls, which actually gets people out the door to vote, show both Hillary and Joe getting crushed by Trump, but especially Joe. There was a 13 point gap in voter enthusiasm between Hillary and Donald in November 2016 with Trump having more, and a whopping 28 point gap in favor of Donald over Joe for 2020 as of July.

So while Biden is doing better than Clinton in who people would vote for polls, he is doing far worse than Clinton in enthusiasm. And that's going to hurt particularly in this election, because instead of just driving somewhere to vote now you have to submit request for absentee ballot, figure out how to fill it out exactly correctly, mail it to the right place by the right date, and all for a guy not many are excited for.

Here, courtesy of Wikipedia:

2016:

National

Statewide

2012:

National

Statewide

And for comparison, here are the current ones:

National

Statewide

Long story short, Trump was much higher in the polls, averaging around 44% in late August with spikes to 49% (41% this late August and spiking to 45% right now) and managed to win quite a few of them, especially those in early September where he crushed Clinton in most polls.

Meanwhile Clinton was polling in the high 40's quite often, but also dropped below 40% in other polls, averaging at 46%. She was far less consistent in her polling numbers than Biden does and fluctuated wildly from poll to poll, who is averaging 49% for late August, though he dropped a bit in the latest polls - but still stays ahead of Trump despite if it in all the polls, something that was never true for Clinton.

In fact, the last time Trump beat Biden in a national poll was on April 3rd, 45 to 43% He managed to match Biden 4 times after that, but never managed to get past him again, while Trump and Clinton both had polls with them ahead at least once every week.

538 had the polling average at 48.0% for Clinton and 43.1% for Trump at August 31st.

https://projects.fivethirtyeight.com/2016-election-forecast/

To see it: scroll down to 'How the forecast has changed' and click on popular vote.

Smallest of comforts: the insane RNC this year does not appear to have provided the usual small polling bump that the DNC did.

https://www.vox.com/2020/8/30/21407646/trump-approval-poll-rnc-abc-news-ipsos
https://www.ipsos.com/en-us/news-polls/abc-republican-national-convention-2020

Well the DNC caused Trump's approval to hit 52%, a record high. Maybe conventions are becoming counterproductive lol.

Is there any system to prevent people on requesting a ballot, receiving it home but still going to vote in person?

538 about Minnesota: https://fivethirtyeight.com/features/why-minnesota-could-be-the-next-midwestern-state-to-go-red/

As far as I know, in most states, nothing will hinder you to vote in person, provided you didn't send your ballot beforehand. You can still choose which you want to do, but you can't do both.