Does my jokingly saying, "What do you think if we make an idol for her and worship it," constitute apostasy that necessitates performing ghusl and uttering the Two Testimonies of Faith, and are my prayers invalid?

We ask Allah to accept your repentance. As long as you have repented and regretted, that is sufficient for you. Investigating whether apostasy occurred or not does not have significant implications, for even if it were assumed to have occurred, your regret and repentance are sufficient. Ghusl (ritual bath) is not a condition for the validity of entering Islam. The preponderant opinion is that it is not obligatory at all, unless the person commits something that necessitates ghusl. Uttering the two testimonies of faith (Shahadatayn) has already happened with your tashahhud in prayer. And the phrase you uttered—despite its reprehensibility—is not explicit in wishing for or condoning disbelief (kufr). The default is that a Muslim remains Muslim, and ambiguous phrases do not render a Muslim an apostate.