Does the MTK preloader reside in the same flash as system or data?
It's present in the Boot ROM, not a directly visible partition in the eMMC.

Source: MediaTek details: SoC startup

The Boot ROM will be embedded inside the processor chip in generic chipsets. (read more) (no idea about MTK)

Further Technical details on working of Preloader: MediaTek details: Partitions and Preloader