From what i have read there is no way of archiving a sealed bucket. Thats because a bucket only gets marked for archiving when its beeing sealed it can't be done afterwards. The only difference for a bucket that is marked for achiving and one that is not are that the archiving one will be copied to nfs as soon as possible after it beeing sealed. Both remains on the appliance and is searchable until it ages out and gets deleted to make space for newer events. To be clear Archiving doesn't delete anything it copies it to NFS to be saved and imported later if the data should be needed.
The retention period of the Partitions will not help you with archiving. It will only partition the data from and then save them for different amount of time. Say that you have two Partitions. On that collects informational logs and one that takes the rest. If you set the retention period for the informational logs partition to two weeks it will age out any bucket in that partition after two weeks or if the log insight appliance runs out of diskspace. The other partition could have another retention period or not anyone defined and the buckets in that partition will age out according to that.