He's still pretty confused why the Visser keeps, what, asking him for advice? Maybe that's just what Yeerks do, it is the thing Matirin said, turn all the host's hard-earned capabilities and resources toward destroying what they care about. And, of course, the Yeerk is protecting what he cares about too - maybe it did all happen the way he said, and this is just the most epic, pointless, wasteful tragedy he's ever seen...
If Leareth wasn't currently Yeerked, he would try for someone with the communication spell. He doesn't know who's on the ship, but someone must, they can relay a warning. However, his Gifts are locked out, and whether the conditional set-command undoes itself automatically or not when the Yeerk leaves their head, one, seems to depend on the person, maybe their state of mind or framing around it (Nayoki was so annoyed about this, she hates inconsistency), and, two, if it does unblock his Gifts, he's compulsioned to Final Strike and this isn't the situation it was meant for but he's not sure whether he could avoid it.
...He would try a radio broadcast at Earth, he thinks. Just blast it at Alaska, maybe Canada - he doesn't know where Matirin is, that was the point... And there might be time for a mage to get a message through. And maybe try a really low-tech method from space, if they don't have sensors working they probably still have windows, can the Pool ship...flash its lights or something...do humans have codes...