I am a Voting Location Manager in Ohio and have run elections for about 5 years now so jumping in here.
My first point of advice to anyone who is concerned, is if you truly are, take the 3 hours it will take in any state to get trained as a poll worker. It will likely end any of your concerns.
To the OP absentee ballots not showing up is not abnormal and not evidence of anything except the postal system loses mail. I saw one story of someone finding absentee ballots in a storm drain and it was like a dozen, not some vast haul, and the investigation found it was likely someone in the neighborhood who took them out of people's mailboxes and dumped them in the drain. Given the widespread belief that college students would lean Democrat IF, and it's a big if, something nefarious happened it's likely someone locally refusing to deliver the ballots to lower Democrat votes. The solution to that is to monitor your ballot and deal with not getting it far ahead of the election, or participate in in person early voting which is pretty common in most states. As a poll worker I cannot vote on Election Day. I do not work in my precinct, so I must either do absentee or early vote. Never had a problem. I can monitor my absentee ballot for all parts and if it gets lost in the mail (takes more than 3-4 days after it says it was mailed) I file another application online. They will send you multiple ballots because they know they will only count one so sending you multiples does not cause any issues.
To the point by someone above on the Ohio process, yes if you request an absentee you can vote a provisional ballot on election day. You cannot show up with your absentee ballot and ask to vote on the equipment. I have no way of knowing the above, if you asked for multiple and already sent one in and the one in hand is just another one, so to avoid that you must follow systems that are all manually verified. I have no reason to believe all states are not similar.
There has never been any evidence of voter fraud at scale to impact the results of an election. This is the key point. There are incidents that happen. The fact that we know about them verifies the system works and should increase your confidence not lessen it in much the same way that you know murderers are in jail should increase your confidence that police catch murderers not make you concerned that the crime rate is rising. We caught the fraud. Also the stories of people voting who should not, are I would contend 100% people not understanding the system. As a poll worker it is my job to defer to the fact you should be able to vote than to find reasons you cannot. So if you insist that you are registered and have the proper ID (in Ohio that is now basically a driver's license, state ID or passport) if you are not in the rolls I let you vote provisionally. The people on the news claiming that their friend who is ineligible voted are those who think that because I let their friend fill out a ballot they voted. There are multiple checks on the back end at the BOE to verify that vote before it is counted. If they cannot find the voter registered anywhere in the state that ballot is not counted. The misinformation that exists is that people just assume because they filled out a ballot it is counted. It is not. It is only counted if we are 100% sure it is for a registered voter and that voter has not voted another way.
On the college student front, yes it is illegal to vote in two states. This applies to retirees who have homes in Florida and somewhere else too and there are cases where those have been prosecuted. As a voter you pick one. Your student applying for absentee ballots in NY and IL is technically OK but if they had returned both and voted for president on both that would be an issue. Since elections are run by states there is no national database to cross check but if they are caught it is illegal because they are casting multiple votes.
Happy to answer any other questions, but stop worrying about massive fraud. It does not exist and is propaganda designed to impact integrity of the system and those of us who do this hard work and adminster it really do not appreciate the distrust all this lying has caused.
