Free seat selection does apply to OW Sapphires and Emeralds, as well as all passengers in the same PNR. It also applies to economy redemptions no matter tier.

Advanced OLCI is not newish. I've never bothered with OLCI well in advance because of schedule changes. If one has free seat selection as a result of tier level, it is advisable to choose the seat prior to OLCI. I am not sure if the system allows further changes once the boarding pass has been issued. Perhaps it requires a phone call.