How does MS SQL Server authenticate MSDTC connections when using SSPI?
There are two Distributed Transaction Standards Supported by the DTC. XA Transactions and OLE Transactions and recovery is implemented differently.

XA Transaction Recovery Process use xa_recover_entry of GetXaSwitch to get a list of xid from the transaction resource manager (SQL Server is a resource manager) that it then uses to rollback or commit the prepared transactions. I assume that running as NETWORKSERVICE is enough to do that based on what is documented in Disabling TIP, LU and XA Transactions


  In Windows XP , Windows Server 2003, and subsequent versions you can
  disable XA transactions to help prevent the security risk that arises
  when a user-specified DLL, used by the DTC to communicate with the XA
  partner's transaction manager, is loaded directly into the DTC
  process. This situation exposes a resource manager's databases to
  serious data corruption and can cause Denial of Service (DOS) attacks.
  The disabling of XA transactions helps to protect the DTC from this
  DLL attack.


The recovery process for OLE is implemented a bit differently. The resource manager has a list of prepared transactions and is responsible for reenlisting them in the DTC. Have a look at the Performing Recovery section of Implementing DTC Interfaces.


Not sure I actually provided a definitive answer for you here but I thought this was too much for a comment.