My father wants to marry me off himself at home without the registrar. Is it valid, and can it be registered later?

Your father is indeed the wali entitled to say the ijab, but the law in Malaysia requires the akad to be conducted before the registrar or with the registrar's permission. An akad without permission to marry and without the registrar present can be an offence, even if its pillars are complete. Registering it later may also require proceedings in the Syariah Court. As a result, your marriage certificate, your children's birth registration and your rights all become more complicated. If your father wants to say the ijab himself, start by applying for permission to marry as usual. Then ask the registrar to attend while your father says the ijab. This protects your father's right as wali and your rights in law. See a father saying the ijab.