A) Will I need 1h proficiency to make a 2h effective on horseback?
B) Is this viable?
C) How's my current build plan, given that the 1h, 2h and pole proficiencies are already benefitting from melee wpf sharing?
A.) I don't believe so. If you were using a Longsword or something with a shield, yes it would be 1h.
B.) It is certainly viable to do this build, though 21/21 with full Ironflesh may be more worthwhile than the extra PS, especially if you are wearing heavy armor (bearing in mind the synergy between HP and armor).
C.) Unless you want to use a 1h weapon, I would just split the prof between 2h/pole. You may not need all 7 WM to do this effectively and could keep the 24/21 build with 2 IF 5 WM.