at the point the majority of the helpless yeerks in the pool without host would have been captured beyond the help of the ones with host, leaving them few in number, and by that time a way of detecting contollers would be made, a miniature device to detect kandrona energy they could replace the military and president controllers with yeerk peace movement or yeerk morphs by any members or allies of the animorphs, the yeerks in the peace movement would agree as a alternative to destroying the yeerk pool killing 100,000s of them in the scenario. The chee would be great allies, they could have the know how in consructing a kandrona generator, if that is not possible the animorphs could find the nearest alien species to barter for materials for making a kandrona generator, the animorphs could leave as a important ranks of the yeerk empire and peace movement hosts with peace movement yeerks, develope a code for hosts and the animorphs to communicate with yeerk peace movement yeerks so that they would get in the right host.
by then there are means to detect who has yeerks in the body they could modify the filters in to small devices, miniature x ray machine, miniature kandrona energy senses,with the chees help and ax's.
Make a deal with the goverment and miltary by then who should be either free or yeerk peace control so that the a group of trusted soldiers and officials would help in hiding the yeerk holding pool, and offer soldiers to defeat the yeerks in exchange they will get alien technology and can used the friendly or captured yeerks to understand them, gain higher intelligence, learn faster and the water filter benefits to supply a endless supply of fresh water for third world countries the hippies, tree huggers and animal activists may be kind enough to understand yeerks and go along with the risky but beneficial plan of limited bloodshed in defeating the enemy. If the animorphs can give sensible priorities to their morals like they should focus more on killing as least as possible issues than it is wrong to morph "sentient creatures".
They could morph the yeerks and gather knowledge and skills from other people like scientists, politicians, military,business man, capitalists,lawyers and more to blend in and use their resources around them to their advantage. and other useful information they could gain to have new insights and planning skills to learn assuming that these people would know of the invasion and be accepting, which by then they are free or have been registerd to the yeerks as controlled but withe recruits morphing yeerks or free or under the yeerk peace movement. Even better the yeerk prison should be in another country free of yeerks. While this is going on the animorphs could apply their knowledge with help from the military or gov't start recruiting soldiers in other countries secretly and finance their operations to make beam weapons dracon beams and other useful devices in a factory of war with ax's help it could happen.
Getting the materials to build the kandrona ray is one of the hardest part but if the yeerks use infiltration to win, why shouldn't humans . They could steal a ship from the yeerks or some other way to get to the alien species who can provide them with materials for the plan, fake a crash, and get out with the materials or ask advice from the chee or yeerk peace on how to acquire them. by this phase the yeerks would think the miltary and gov't is under their controll and infiltration but in fact it is the other way around. the yeerks would be driven out of the gov'ts and miltary heads imprison with portable kandrona rays, The yeerk will be replace by other peaceful yeerks or the recuits in morphs or other soldiers. The yeerk prison would of course be built in another country far from the interests of yeerks. Once it is finished by then their should be a sizable army, the yeerks would be infiltrated already ready for the next phase.
The chee should gather biological data on the yeerks, hork bajir , taxons. so chemicals can be used to make effective sleeping gas them knocking them out, a drug should be made to make the yeerks susceptible to hypnotism or brain washing to get them out of the host, it will be helpful for some reasons.
1. The 3 days of waiting would be shorten.
2. They could imprison the yeerk instead of killing it , and get useful information.
The chee could help turn or sabotage the defenses of the yeerk base or turn it on the yeerks. cut of communications to the empire and outside help.
The next phase is to capture visser three or other high ranking yeerks should be captured their hosts infested with axilliary animorphs yeerks, capture as many controllers as possible, replace them with the yeerk peace yeerks and the other recruits to infiltrate the yeerk base. then have visser three order that the majority of the yeerks without host have to be transfer and put on a ship., hold a meeting with the army at a huge room laced with dracon beams defenses and hunter robots, and sleeping gas. The chee would cooperate as the traps are mostly non lethal and modified. The army and the recruits could fight guards guarding the yeerk pool , free the hosts and give them weapons the with the animorph already in the ship with the helpless yerks, the animorphs would transport the majority of the yeerks into the yeerk prison in another country, mean while the traps are set on the majority of the yeerks army incapacitating them and the entrances to important rooms like armory are sealed of with bombs creating a barrier of rocks that no one can get into for a long while without any way of calling for help. with the majority of the yeerks captured the campaign against earth is crippled, the remaing yeerks with hosts are too few and scattered to offer any real fight, so their alternative is to surrender. As a bonus before the scenario happens the animorphs could further weaken the yeerks by helping the taxons by some means to convincethem to swith sides.