So my thoughts are that the first major butterfly an infested Rachel would cause (regardless of the attitude of the yeerk) is that the morphing cube is never lost at the construction site. She hangs onto that thing without fail and David never enters the narrative at all as a result.
At this point we hit a divergence, dependening on the core motivation for this yeerk to believe a symbiotic relationship with hosts rather than a parasitic one is desirable.
On the one hand, if our yeerk supports symbiosis because they're a patriot, have no particular care or respect for the host species, but have recognized the war has become detrimental to yeerks as a people and is just a tool to concentrate power into the hands of a few warmongering megalomaniacs? Then they probably vanish the cube to begin discretely giving like minded yeerks morphing powers with the end goal of enacting a coup to take over the yeerk empire. Meanwhile they're using their position as Rachel to try and keep the Animorphs out of the war entirely, to protect yeerk lives, and that probably works for the most part because Marco and Rachel being in agreement would sway both Cassie and Jake. In short, the series ends up with just one protagonist - Rachel's yeerk, instead of five or six.
On the other hand, of Rachel's yeerk supports symbiotic coexistence on moral grounds, then we get a much more interesting story, because I think this is a yeerk who wins Rachel over before they ever meet Elfangor.
Coming out of this traumatic experience where her cousin Tom has basically kidnapped her with a voice in her head saying "I know this is scary, I know you're confused, I'm going to help keep you safe, and explain everything once we get home, but I need to focus right now so we can convince them to leave your mom, Jordan, and Sara alone" would get some major points for the yeerk, enough to buy time to explain everything, and Rachel would 100% be on board to help this desperate rebel alien with their underground resistance to save both their races.
I see this as a Rachel who is mostly living her life with the yeerk as a helpful passenger, and yields control without resisting when needed, and has control taken when her temper is endangering them, which would, I think, be a helpful experience for Rachel, to have an ally that can force her to take a breath, count down from ten, and really think before just reacting.
This is a yeerk who comes clean and apologizes to the other Animorphs that first time in Cassie's barn, and emerges from Rachel to morph (having used the cube with Rachel's help once they were home) and the Animorphs have a sixth member who's an alien advisor right from the start.
Marco finds out about his mom, Jake knows about Tom, the extent of the pool's security is quickly revealed, and the early missions have a lot fewer 'victories' that actually just get hosts and yeerks killed and make things harder on everyone without actually slowing the invasion much at all. There's a lot more spying, sabotage, and intelligence gathering, and stealing a portable kandrona generator probably becomes a major early priority because it lets them target the main pool while still having an option for yeerks who're willing to let the war end.
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