Why did Konrad hasten Regretless's return? Simple, regardless of whether by his hand or Celestial Slaughter's so long as Cloud or Night faced the threat of extermination, alarmed by his Supreme Will, Regretless would awake. And even if they could stall him for three to five years, the time of his return undoubtedly neared.
That being the case, using him to source Celestial Slaughter's abilities undoubtedly was a wiser choice. If knowing the enemy was the first rule of warfare, on the war for information, Celestial Slaughter currently held an indisputable advantage. They knew too little of him.
With Regretless's typical conceit, not putting Celestial Slaughter in his eyes was natural. But unfazed, Celestial Slaughter curled his lips into a smile.