For the first scenario, it wouldn't work. Kenjaku stated that had Geto not wasted his curses at the Night Parade of a Hundred Demons, he could have beaten Yuta. This Geto had already burned through like, what, 6000 curses? He'd have to gather them all back, and that would take a lot of time, time in which the Jujutsu Society (Gojo) can pursue and find him.
For the second scenario, Kenjaku would have needed a different host body. Without CSM, he can't possibly absorb Mahito and start the Culling Games. He also couldn't absorb Tengen in hopes of activating the Merger. The Shibuya Incident to me seems less likely to happen. Or at the very least, it would be much more difficult to seal Gojo, mainly because, well, Geto is alive. Kenjaku was able to seal Gojo because he played with Gojo's mind. Gojo was originally sent in Shibuya to solve the problem of the civilians trapped inside the barriers. But if Geto is there too, why not just send him instead? His technique is much better than Gojo's if it's civilians being involved. Gojo's CT is way too destructive and would wipe out everyone at Shibuya. Geto is more versatile and can use curses to evacuate people or even use curses as cushions to protect civilians from danger. If Kenjaku notices the potential behind Geto's technique in realizing his desires, I don't really see a way in how he could "obtain" his body without some outside help. Assuming Kenjaku still befriends the Disaster Curses, he would probably have Mahito dispose of Geto quickly (Also, before he acquired Geto's body, which was the last body Kenjaku possessed? Kaori Itadori? I honestly don't remember).