Honestly Throwing Knife does not need a dagger. You are not throwing your dagger, you are throwing knifes, attacks with bow/crossbow don't afford you to have a stock of arrows/bolts, so the same could go for those throwing daggars. In the end, a weapon for close combat and a weapon for throwing have to meet different criteria to be effective anyway.

But a spear is a piercing weapon, not a cutting weapon, so cutting someones tendrons or even his throat would be pretty hard to achieve with a spear. But there are many warfare skills that work similar too Bull rush and would work with a spear as well as with a two-handed strength weapon.

In general Polymorph is a multi-class skill tree. Some skills are more for warriors like tentacle, some are for mages like Medusa, some a typical scoundrel like chameleon, but most are not that specific at all.

But it would not make much sense to have a magical attack like Medusa head scale with Finesse for example. Though I think it would make sense to be able to backstab with any kind of piercing weapon and not only daggers, so spears and arrows aswell though it would make archers even more broken, so spears alone would probably suffice.