I think the Scarlet Centurion was established to be an identity Rama-Tut used before he became Kang, back in AVENGERS ANNUAL 2, so when Kang and Immortus were revealed to be the same guy, he was already in the mix.
And by the time Moondragon returned in AVENGERS, Hellcat had already turned up in the brief Gerry Conway run of DEFENDERS, and the explanation (such as it was) of how she'd departed from Titan had happened there.
I think the Scarlet Centurion was established to be an identity Rama-Tut used before he became Kang, back in AVENGERS ANNUAL 2, so when Kang and Immortus were revealed to be the same guy, he was already in the mix.
And by the time Moondragon returned in AVENGERS, Hellcat had already turned up in the brief Gerry Conway run of DEFENDERS, and the explanation (such as it was) of how she'd departed from Titan had happened there.
I had a feeling you would know what happened with these plot points.
The Marvel Universe -- it's all stuck together!