WDS Station + Dumb AP (WDS Client/Repeater)

Installing and Using OpenWrt Network and Wireless Configuration
2
  1. Yes. Example configs: Proper configuration of 802.11k and 802.11v - #107 by patrakov. Note that it only works on non-DFS channels.
  2. I have not touched it, and it is off by default.

STP is useful only if there are loops in the bridge topology, which is not possible for an architecture with repeaters.