“Gravekeeper’s Assailant” changes Attack Position to Defense Position or vice-versa. It cannot flip a monster face-down because that is not a change of battle position.
You make the change of battle position due to “Gravekeeper’s Assailant” at the beginning of the attack, so you can change the battle position even if the attack is then negated by “Magic Cylinder”, etc.
If “Imperial Order” and “Necrovalley” are both in play, “Necrovalley” is still on the field so you can still activate the effect of “Gravekeeper’s Assailant”.
The effect of “Gravekeeper’s Assailant” targets 1 monster.
If “Gravekeeper’s Assailant” attacks, and activates its effect to change the battle position of the opponent’s “Spirit Reaper”, then “Spirit Reaper” is destroyed by its own effect. Then a replay occurs because the potential attack targets on the opponent’s side of the field were changed. In this case, “Gravekeeper’s Assailant” can attack again, and can activate its effect again.
If “Necrovalley” is on the field, an attack is declared with “Gravekeeper’s Assailant”, and the effect of “Gravekeeper’s Assailant” is activated, and “Mystical Space Typhoon” is chained to the effect to destroy “Necrovalley”, the effect of “Gravekeeper’s Assailant” is still applied because “Necrovalley” only had to be on the field to activate the effect.