Yeah, time to get the fraud is either in real-time or matching signatures.
Most cases where elections get overturned is when a signature match is ordered by a judge and enough spoiled affidavits (That is the envelopes) are exposed.
Not you but to other people, States are required to hold the affidavits (Including mail-in-ballots) for a set time (Usually 2 years involving Federal elections). A thrown out envelope will count as a false affidavit.
Right. One could imagine a scenario similar to what played out in North Carolina last year, where the Republican won but there was evidence of illegal ballot harvesting, so they held a special election where a different Republican won by a bigger margin. But that scenario would almost certainly require action by the state legislatures, and it is almost certainly too late now, particularly for it to happen in enough states to place the outcome in doubt.
I think people placing their hopes in the courts have a misunderstanding of what happened in Bush v. Gore and buy the Democrats' story that the Supreme Court elected Bush. No courts were ever going to go so far as to overturn the result of an state's election. It was always going to require the intervention of state legislatures.