The real harm in this isn't even letting Democrats cure the votes and Republicans not cure them.

They were opening mail-in ballots and looking at them illegally and no doubt without any observers.

They could have opened the secrecy envelopes, replaced Trump ballots with Biden, and then sealed them up in new envelopes. The secrecy envelope is just a plain envelope with an anonymous ballot in it. Nobody would ever know unless they were there to witness it.