Last Question (I think): you both refer to "THE cold backup."
Because you started the thread with the title "Cold Backup". We assumed that you knew what you were talking about.
We use a diff backup
Which is not a Cold Backup.
I assume you mean "copy over all files since the last full backup prior to the date that data corruption began"?
That is "copy over all the archivelogs since the last full (cold) backup upto the time of the data corruption".
This thread has subsequently gone down the path of RMAN Backups from the Standby site and you don't have any backup old enough to use.
Therefore, you cannot use any strategy -- except, somehow, manually fix the data in that table.