I have gotten the impression that Yeerks are fairly soft. Therefore, I would have Cassie (the best morpher) assume a small tough life form, then do a careful demorph in such a way as to exert maximum pressure on the Yeerk while minimizing pressure on her body.
Er. I think that would actually increase V3's survival chances, and decrease Cassie's. Yeerks being so yielding, V3's body could just squish out of the way, whereas Cassie's tougher body could potentially shatter. You either bend, or you break.
I'd definitely have any potential tiny-assassins go after Alloran's body, rather than V3's Yeerk self, for that reason. The Yeerk would be too hard to kill at that scale, it'd be like trying to murder Jell-o.
I just thought of a slightly gross idea. It'd work best if Ax did it. Morph an insect of some kind, like you said. Something sturdy, maybe a flea. Sit at the top of a blade of grass in V3's feeding meadow. Wait until he steps on you and sucks you up one of his hooves (Ax mentions accidentally eating snails and worms, so we know that Andalites can accidentally ingest insect life while grazing). As soon as you're in his stomach, start demorphing, and as soon as your tail forms, slice your way outta there.
No idea what the inside of an Andalite digestive system is like, but the Animorphs have clawed their way out of being eaten by a lot of different things, and they usually manage to kill or at least mortally injure whatever the thing was that ate them. So that's probably your best bet.
Wouldn't V3 be able to get rid of dead Rachael by morphing?
Hmm. No idea. Either she'd be absorbed into his body (eww) or else it'd be a case like in #15 with the metal implants.