Not true. Is it possible you missed something in step 3??
that the 'original primary ( failback target ) should have been OPEN ( not MOUNTed )'
Once the databases are in sync you can do a failover again. It will not matter which database is in the Primary role, only that they are in sync.
If I do failback to the original primary, may I yet-again a re-failover to the standby?
The answer is No. Guarantee - if your setup is correct you will not change any part of it because of failover or switchover.
if anyone knows whether or not I need to resetup ( at least some ) parts of dataguard, I would appreciate the clue.
Once this is done the database should sync and the only difference is they are in opposite roles. No setup changes needed.
SQL> FLASHBACK DATABASE TO SCN <YOUR_NUMBER_HERE>