Exit polls were always unreliable, and even more so now. One should not take the “instant” reports based on exit polls as being proof of anything.
Most pre-election polling suggests a Republican win, and usually the polls “lean” Democratic. However the Republicans seemed to be “surging” at the end, meaning that the mail-in and early voting might have missed the surge.
The lawsuits should largely vanish in non-close elections.
Undated but verified envelopes (meaning the voters name is on the outside can be checked) are unlikely to be forged, especially if they have a postmark. One has to ask as to why one assumes that Democrats are more likely than Republicans to improperly fill out the absentee ballot forms (being a Democrat probably indicates poor economic and political judgement, but doesn’t indicate inability to fill out a form).