Well it is nice that they are going to keep helping his dad, but if he keeps being an alcoholic that's just going to come back to bite everyone in the ass in the long run
Not necessarily, they don't really give a shit about test now that they know it was setup due to blackmailing, since it was all about humbling the brother, not about the score or good reputation (they are proudly presenting as villains), and they basically just did get a win over brother by getting Kei back in front of him. If they tried to clear accusations, then Kei would get blame too for lying, and if they no longer care about it, there's no reason to do that.
They can't do that or else it'd make all of Kei's efforts meaningless, since her dad would get exposed anyway and plus she'd get in trouble for planting evidence