Is it Islamically permissible to use the "Mardouma mixture" — composed of iron, cloves, henna, salt, and gallnuts — to straighten curly hair, especially since gallnuts make hair dark black and my hair is already black or very dark brown? And can gallnuts be replaced with another natural ingredient if the mixture is deemed disliked or forbidden?

If your hair is black and will turn to another level of blackness, then it is permissible to use the mentioned mixture to soften it, as long as it is pure and permissible.

Sheikh Ibn Baz was asked about using a mixture to soften hair that contains an herb that dyes it black. He replied that there is no harm in that if the hair does not have any gray. However, if there is gray hair, it is not permissible to use anything that dyes gray hair black, due to the Prophet's (peace and blessings be upon him) saying: "Change this gray hair, and avoid black."