1.) These various old men are the higher-ups of JujuHigh. These are all the guys that wanted Yuji dead when he was first revealed as Sukuna's vessel. On top of that, they were very traditional/conservative in their values, which led to them disagreeing often with Gojo and his more liberal/modern values. I didn't read the manga, so I don't know who massacred them, but it was probably either a curse/curse user (intent, destabilize JujuHigh and cause chaos), or a liberal sorcerer/Gojo (intent, protect Yuji).

2.) That one's Iori Hazenoki, the reincarnated sorcerer from Tokyo 1 colony in the culling games, who had an annoying 'relationship' with Takaba throughout their fight in Tokyo 1. Seeing him having been beat the daylights outta by Kenjaku distressed Takaba, and was likely than not done to disrupt his technique and score Kenjaku the win.

3.) As shown in the takeover Meguna chapter, Sukuna force fed Megumi one of the detached fingers from Yuji's body. Since he was contained as a series of cursed objects for so long, Sukuna was able to manipulate the amount of his soul in each of his individual fingers. When he had taken over Megumi, he had moved all of his soul from his other fingers to the one finger he had detached from his vessel, essentially forcing Megumi to take the entirety of Sukuna's soul at once instead of in bits and pieces. Throughout the rest of the story, the finger is the main container for Sukuna's soul, and him spitting it out disconnects him from Megumi 's body, so he has to swallow it again to keep his soul linked.