How can one get rid of masturbation, especially with the inability to marry, and are there any proposed solutions for dealing with lust, and what is the ruling on "milk al-yameen" (concubinage/right-hand possession) in this context?

Masturbation is forbidden, and there is no permissibility for performing Istikhara (prayer for guidance) for the purpose of committing forbidden acts. Istikhara is for recommended and permissible matters. However, if someone is compelled to practice it—such as one who fears falling into adultery or illness—it is permissible for them to do so out of necessity, as some Companions and Tabi'in (early Muslims who followed the Companions) permitted. As for slave-girls, they do not exist today.