I don´t think he knew that Ruin had so much power over hemalurgy. Especially while he was confined in the "Presevation prisionTM" otherwhise he would never have created the inquisitors/kollos/kandra the way he did.
He needed the metal to pierce his skin so that he could burn it for youth... remember, he used the atium as an alomantic and feruchemical metal, so it needed to be outside of him to charge and inside of him to burn
there has to be intent to be hemalurgy, so no, the metal doesn´t have to be hemalurgically charged, but the person spiking (or Ruin himself) have to be activaly trying to use hemalurgy
EDIT: trying to quote the parts i was replying to in each paragraph... but can´t seem to make it work
EDIT2: Actually, i´m not sure if the metal HAVE to be outside the body to feruchemically charged... does anyone know?