Is it possible to disable the internal DC/DC converter and supply the core voltage of 1.2V externally?
Yes, this is possible.
The Schottky diode (D1) provides a measure of protection when VDDIO is below VDDC by limiting the voltage (VDDC – VDDIO) to the forward drop of D1.
The Schottky diode D1 as illustrated in the Figure is optional, e.g. a recommend part number with a ultra-low forward voltage drop is the SBRT05U20LPS from Diodes Incorporated.