I hope you don’t mind someone from outside the book club chiming in.
I haven’t read this specific Dragon Ball manga, but that must be the part where Oolong is kidnapping girls, right? I think in the first panel, the author had Oolong go out of his way to mention 知らない人になってしまった since there’s no way Oolong would know who Zarbon is, and that form probably isn’t exactly what he was going for. In the original manga he just transforms into a generic attractive guy, so I see it as emphasis on the fact that he’s turning into a character that a lot of readers might recognize even though that character hasn’t been introduced yet. So more than anything it’s kind of a fourth wall break to accentuate this gag that’s referencing another story arc.
For the second one, assuming you’re talking about the ellipsis, I think he’s trailing off saying something like 何とかしなきゃ or one of its variants, as in “i/we gotta do something.” So the full sentence might have a nuance like “we gotta do something, while he’s in his current form.”
For the last one I think it’s important to mention that 知らない can sometimes have this nuance of something like “I don’t care” or “that has nothing to do with me.” I can’t explain it well, but on jisho.org you’ll see that the entry for 知らない includes “i don’t care” or “that’s irrelevant” as translations. So you could interpret that sentence as “Even if you regret it, it doesn’t matter to me.” Xても知らない is an expression that’s commonly translated as “don’t come crying to me if you X” or “don’t say I didn’t warn you.”
At least I’m pretty sure that’s what those mean. Someone can correct me if I’m wrong.