Question on CPU, Fallback and more

[MaxS1996]

For a student project I want/need to combine a RISC-V CPU with a custom DL accelerator and am in dire need for a software stack.
ONNC looks really promising as I am only one student and can't do a lot compared to bigger more skilled groups, but I have a few questions, which I could not answer after reading the documentation:

• How hard will it be to run operations on RISC-V (given that I have a standard compiler for that target)?
• Can ONNX operations on the local CPU, if the accelerator does not support them?
• How hard is it to add custom datatypes?

Thank you for your help (in advance)