Ok so it's safe to say that Sukuna transfered all of his power into that finger.
I don't think what Sukuna saw in megumi was the potential to be his vessel because he said
It's a gamble from now
So he probably just took a risk here, but still it's important, to his relation with Kenjaku imho i think this means that he didn't trusted what Kenjaku was planning to do with him and decided to switch over to Megumi.
And the fact that he treated Hana gently also mean she's important to him ( not sure if the translation is correct but i remember he said he wasn't allowed to kill, but nothing about hurting )
Damn can't wait for next week chapter