Answer
A version of the IFS EE Extension released starting with Apps10 UPD6 was specifically modified so it can be installed without requiring local administrator rights or elevation on the target machine, resolving exactly this kind of 'works manually as an admin account, fails via unattended SYSTEM-context deployment' symptom. If your environment is below UPD6, either upgrade to UPD6 or later, or contact IFS support to request this fix as a standalone single patch rather than a full update, since installing without elevation isn't possible on pre-UPD6 versions of the extension.