How do manually switched native move collections work and do they have a maximum lifetime while "Ready to Switch"? (4340387)

When processing a manually switched native move collection, the MAgE (agent) will add the -SuspendWhenReadyToComplete parameter to the New-MoveRequest command that is issued to begin the migration of any given mailbox.

The MAgE leverages the native capabilities of Exchange.  When the migration administrator decides to switch the mailbox, the agent will just resume the move request, at which point the Exchange Mailbox Replication Service (MRS) will handle all the delta changes since the request was suspended.  MRS can even perform the entire move-request again from the beginning if the target mailbox has been somehow deleted.  Therefore the entire process can be thought of as Migration Manager encapsulating the standard native move request functionality within Exchange.

The ReadyToSwitchHoldingPeriod Migration project parameter does not apply to Native Move collections, only Unified Mail sync collections.
There is no maximum life time for the suspended move requests.  The only consideration is that the target mailboxes do not have an owner until the move request completes, making them vulnerable to removal by the store if they reach the retention period set for the specific mailbox database they reside within.  This is 30 days by default.