Okie dokie. In the last Cassie book before 50, she thinks the Yeerks getting their hands on the cube would be the worst thing ever. I really don't think anything happened between those books to change her mind.
This is a very good point that I hadn't thought of--and it bugs me that I didn't. It kind of makes Animorphs 39: the Hidden kind of pointless, doesn't it? I'll have to double check to see if she actually said that, but it's not like I don't believe you. 44: the Unexpected?
Second, I'm not sure how it was the deciding factor at all... Yes it caused some Yeerks to turn traitor. I don't think they needed to hand an escafil device to v3 to do that. In the end, she's partly responsible for Rachel dying. This was not a well thought out plan that she consulted with the group over... It wasn't needed to get aboard the pool chip, if I remember correctly.
Here is where I have to disagree. I feel that handing the morphing cube over to the Yeerks was the game changer.
1) It convinced a lot of Yeerks to go traitor, like you said. You were correct in saying that the Escafil device wasn't needed to do so. I'd say it made it EASIER.
Think how fragile the Yeerk Invasion on Earth was with Visser Three messing things up--constantly giving his subordinates reasons to jump ship and plot his death-- and the Yeerk Peace Movement was helping the supposed Andalite Bandits. Then, of course, you have the ambition among the lower ranks and infighting between the Yeerks themselves. Image how much easier it would be for the common, low level Yeerk to prove himself when he has a morph capable host? It would be enough to convince the Council that, maybe said Yeerk would be more competent than old deranged Esplin.
2) It convinced the Taxxons to switch over to the "good" side.
This was crucial. If I remember correctly--from what I glimpsed at in Animorphs 54: the Beginning-- it was what let the Animorphs gain access on to the Pool Ship--along with the Auxiliaries and General Doubleday's team.
Even if it did make all the difference, which is debatable, it could have been done better.
Agreed. It was a risky move and could have easily blown up in their faces.