Parker is the WBO #1 contender. Wlad is the WBO #2 contender. The title is vacant. There's no point in including Andy Ruiz now.
Joshua is meant to complete IBF his mandatory against Parker by January 9th, and there is no guarantee the WBO will sanction Parker vs Ruiz. Surely Joshua vs Parker makes the most sense.
Yes! If Wlad loses to Parker, then he's finished and wouldn't have had a chance vs AJ or Fury in the rematch. Parker is still green, but a top-contender already. The fight will show where Wlad stands today in the HW division.