That is what needs to be proven, otherwise it is an unsubstantiated assertion.
We have the Feast of Unleavened Bread and other ceremonies that require the removal of leaven from your house.
Yes, and, where does the Scripture teach that "leaven" in the context of the Feast is something negative? That's your assertion, now prove it. While you are working on that, please explain how it is that if leaven is "always a negative", why does God permit its use in other religious ceremonies with offerings to Him made with leaven (cf. Lev. 23:17; Amos 4:5)?
. In the New Testament we have Jesus warning his disciples about the leaven of the pharisees (i.e. their false teachings.)
Yes, and we know what Jesus meant from the context. "Beware of the leaven of the Pharisees, which is hypocrisy." That doesn't prove your point.
Leaven is always to be avoided.
See references above. All you've managed to do is prove that you can stretch the Scriptures to say what they do not say.