I posted this in another thread a couple of days ago:
Washington state first began voting by mail in special elections in 1983, began permanent absentee voting by mail in 1991, and virtually all counties switched to all mail ballots in 2005. Other than some races not being decided until two weeks after election day after all the ballots trickle in and are counted, it has been a pretty trouble-free process. I have voted only by mail since 1991. Starting two years ago, you don't even have to put a stamp on the ballot. You can mail the ballots or drop them into a collection box. We usually drop them in the box since collection boxes are outside libraries, amongst other places, and I go to the library often.
Pretty similar to what I was doing in CA before I left. You just get your ballots without asking.
I'm still not totally familiar with Idaho methodology, and we're apparently changing right now due to COVID. However up until now, I have voted absentee. To do so, I have to go to my county website and download a form that I then mail or email to them. I have to specify that I want absenteee and also which elections I want to have absentee ballots for, sign the form and send it in. IIRC, I didn't have a choice of more than three upcoming elections. After that, you have to download an updated form and start all over again. Again, they are apparently changing things now, and it sounds like they are moving closer to the CA/WA method.
As far as mail-ins having an effect on an election, I wonder if WA/CA type mail-in is relatively trouble free because of the makeup of those states? I can't see the mail-in ballots not counted till two weeks later having much of an impact in any presidential election. They will likely follow the very liberal demographics of those states. I would expect the same of any predominately conservative states that have a mail-in system.
Where I think it will get tricky is in those swing states where the counts can be close. Mail-ins not fully counted till 2-4 weeks after the election could definitely impact the electoral college, given the electors of the state give their votes based on the popular vote within that state. If it's a close election, and a swing state has enough electoral votes to impact the final count on election night, things could get ugly.