Yes I agree.
In these crazy times, semiconductor delivery times and prices are problematic.
The ready-to-use PCB module with the MCP2515 & TJA1050 is very quickly available (e.g here
or here ) and much cheaper than the standalone MCP2515 chip. 
Regarding DIY modules - I’ve already implemented a compatible enclosure controller. The prototype on Arduino works fine - there is a post with details somewhere in the forum.
In that topic - I designed a dedicated PCB - I am waiting for delivery, in the simulation it will look like this:
I will use the mentioned CAN module and Arduino Nano - both cheap and easily available.
You’re right - the Emergency button will be easier to implement. I think I’ll try to do it tomorrow.
Once you understand what the communication between the Snapmaker modules looks like, you can implement each module (of course, those that are currently included in the controller software).