The whole fact that a Yeerk morphs WITH the morphing-powered host is a contradiction, to me. I remember upon my first reads it took me some time to question this. For some reason I never really pondered about it when Visser Three morphed, but I think, by the time Jake got infested, I was wondering how it all worked.
It's not like a piece of meat stuck between the teeth like Cassie. It's not like a bullet popping out of the flesh during the morph either. But the Yeerk inside the skull should be treated as a foreign matter in the morphing process. In this process, nothing that belongs to the DNA, say, a bullet, pops out of a wound or, a poison, is purged from the system.
The only way I can see it explaining this is if the Yeerks become literal DNA-extensions to the host via the neuro-connections. It's in tune with the Yeerks assuming the host 100% and a dead-host-is-a-dead-Yeerk thing. In the last moments of death, the Yeerk must break these extensions and Yeerk physionomy automatically works to push itself out.