with my understanding of your question:
you did a failover(not a switchover) to your physical standby DB.
when you want to restart the ex-primary, use startup nomount option.
alter system set LOG_ARCHIVE_DEST_2 =''; <---if you are afraid of ex-primary transferring archive redo log. since you said you have experienced
alter database mount;
then follow the flashback procedure as you wish.
I would test on a test system several times first before doing anything.
These links might help you build your plan.