Is it permissible to arouse one's desires by undressing under a blanket and then redressing before ejaculation, such that no one can see the 'awrah?

The only way to fulfill one's desires is through marriage or lawful concubinage. Deliberately arousing one's desires merely through imagination is not permissible, and some scholars have prohibited it, regardless of whether it leads to the emission of semen or not. As for toying with one's private parts, it is a form of forbidden masturbation, whether semen is emitted or not.