1. The Clinton team had no intention of conceding that night, but after they sent Podesta out to address her supporters at 2AM the whole thing went over very badly in the media. Even most of her own sycophants on CNN, MSNBC, etc. said it was pathetic for her not to address her own supporters.
2. Trump and Clinton never spoke on the phone.
3. The concession was made by someone else -- either Huma Abedin, John Podesta, or even Bill Clinton -- because Hillary Clinton was an absolute wreck and may have been very intoxicated.
I do recall the criticism from her sycophants, even CNN and MSNBC, as you point out, that she owed it to her supporters to make a public appearance.
Have heard reports about her being a basket case that night/morning.
The only proof I have as to the validity of her making a call was the fact that Trump acknowledged her doing so in his victory speech. I would not be surprised if the call was not actually placed by Bill who covered for her.
The absentee ballot issue has risen in previous elections. There has been concern that absentee ballots can be fraudulently franked with pre-election dates. The USPS is den of Democrats.