Is it permissible for the zakat recipient to take zakat if he is one of its eligible categories, without informing its givers? Is using melanin-stimulating creams to hide gray hair considered altering Allah's creation?

The agent is trustworthy, and it is not permissible for him to violate the condition set by the principal. If the donor stipulates that the agent should not spend from the charity on himself, then it is not permissible for him to do so, even if he is in need. However, if no such condition is stipulated, and the donor intends for his charity to reach any needy person, then the agent may take from it for himself if he is eligible. Nevertheless, piety dictates refraining from taking or seeking the donor's permission.

Regarding the dye that permanently changes white hair to black, it is not a [traditional] dye, and there is no harm in using it if it alters the hair's nature so that it grows back black after shaving.