(1) Usually the Incremental uses the Archive-Bit (Windows) or the Access-Time on the Files to determine what to save (using regular incremental) or it reads all Files that were modified since the last Full or Incr (Incr1-X) based on the Date given from the IDB
(1.1) Archive-Bits and Timestamps are independent from the Catalog-Logging
(1.2) The Treewalk-Variable only applies to the first treewalk that collects Size-Info and number of Files to back up that is displayed in the GUI, has no influence on the actual Backup-Walk
(2) Enhanced Incremental uses a local Database to log what is changed. How it exactly works I cannot say. When NTFS Change Journal is enabled most of the work is outsourced there. DP then basically just queries that Log for all changes in the Filesystem since a given Date and immediately knows what to save.