5. Can we upload custom firmware on the SOM-301 module ?

Yes, it is possible to upload your software on SOM-301 module, but not suggest due to the reason mentioned below:

The original purpose of V2Xcast Service and SOM-301 is to separate the V2X communications and applications in different hosts and have a clear responsibility of each party to integrate into a big system more easily. We suggest not to do it when you want to gain the whole advantages of the architecture.