Does REDUNDANCY retention policy guarantee PITR from the earliest backup?
Does RMAN retain all archivelog backups necessary for PITR to any point since the earliest retained backup?


No, at least not if you have incremental backups in the mix:


  …we don’t guarantee a recoverability to a certain point  in time, just, the number of backupsets.


I'm not sure if the same applies if you aren't using incrementals, but there are too many edge cases with using a redundancy-based policy (see this horror story for example). Use a recovery window instead if you possibly can. The article you quote has the same advice:


  The safest option is to use the redundancy window as several backups on a single day could cause the redundancy option to mark valuable files as redundant.