reinstate database without flashback/DB Restore/RMAN

I am trying to get my head around reinstating a database in dataguard scenario, i am very new to DG and hence having issue trying to understand this.

So lets say i have datacenter DC-A and Datacenter DC-B and i have setup DG between the two(Max Perf mode). Now lets say DC-A goes down unexpectedly (and lets assume that DC-A will come back up in 3-4 hours as know we know whats wrong and network team is trying to fix that), also lets assume this happened at noon and my log seq# at primary was 300. Obviously in the mean time i am going to FAILOVER to standby in DC-B and everything is happy. Time passes along and now its 4PM and lets assume now my primary in DC-B is at log seq# 320.

Now at 4PM, my DC-A is back and i want to bring the former primary into standby mode. I can either use flashback(if enabled earlier) or recreate my standby again to do that. My question is, instead of using flashback or rman to recreate my standby(DC-A), cant i just mount my database in DC-A, copy over logs from seq# 300-320 from DC-B on to DC-A, roll forward DC-A and then start my redo apply instead of recreating standby(or using flashback)??

I guess in short i am asking y do we have to use flashback or use RMAN/re-create to reinstate my standby database? is there any other way to reinstate my database(fromer primary).

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