Combining a Linux capable CPU and MCU is not new or that weird... I think a Cortex A9 and Cortex M4 appeared on a combined SoC on the IMX6 platform around 10 years ago.
Unfortunately not. The RP1 controls the stuff on the 40 pin connector and the firmware is locked down. That said, you do have access to the PIO block, which is a programmable state machine that can talk to (I think?) any GPIO on the connector. Pretty fun stuff, good for real time or things with really tight timing requirements.
Plenty of use cases in embedded!