Additionally , if we are okay with the 7.5hrs of down time , is my calculation/extrapolation of total time consumption based on a sample table, for direct alter, accurate? Because, in that case , I was thinking it's less complex and also less error prone to just do it in a single alter command rather than going for multiple steps of detach, alter, attach partition.
Well, it's meant to get you a ballpark figure, but yes, it seems as though you will probably okay, But for something this critical involving production downtime, I would try out the exact command and see how long it takes on a test system. Restore a backup (you have backups I hope) or use pg_basebackup to make a copy of your prod system somewhere.
