To the first comment about poison . . . well oatmeal kind of was poison to the Yeerks, not readily leathal poison but still a sort of poison that screwed them over pretty bad and the book was already walking a thin line as what the Animorphs were planning was pretty close to an act of terrorism (attacking the hostless Yeerks in the pool, many of them noncombatants) further consider that Ax might hate Yeerks but we see in TAC that chemical warfare isn't on the list on Andalite weapons of choice. Besides, consider your comment about rat poison; They, as moles, wold handle rat poison? Even if it were done safely don't you think they'd be a little nervous about that? Maybe that's why they wanted something bad for Yeerks but harmless to humans and earth animals.
To the second comment how would they get explosives onto the blade ship? As far as I can recall they'd never gotten onboard the Blade Ship on purpose as early as seventeen, and I don't think they would have been able to take an explosive with them if they had a morph that could sneak aboard, and what major threat did the ship itself present? The Blade Ship itself wasn't that big of an issue for them since generally it just landed to deliver Visser 3, at no point did it go flying around during a battle sniping them with its cannons or anything (which is what I would have done if I had command of it) a bug fighter could just as easily have done that job and the loss of a Blade Ship might have been serious enough for Visser Three to not only get a replacement (he is Visser Three after all, there shold only be two people with greater requisitioning power than him) and reinforcements, even that open war he was wanting. It could only have benefitted them if he were on it and then they'd have a new Visser to deal with probably V-One or Two since the Yeerks wouldn't send someone ranked lower than their newly martyred hero and Visser One wouldn't have made the same mistakes Visser Three did.
I think overall you have to remember that everything you do will get a reaction from your enemies, if you poisoned the entire Yeerk pool and blew up the Blade Ship you'd make Visser Three's call for open war look that much more reasonable just to flush out the "Andalite bandits" the Yeerks themselves wold be more determined and less likely to surrender since they'd expect no quarter from their enemies.
