Do avionics in general subscribe to MISRA C/C++ or do they go even further with an additional (or different) approach?
Depends on the company in my experience. I've seen some suppliers that basically just wire up the diagram in Matlab/simulink and hit Autocode. No humans actually touch the C that comes out.
Honestly I think that's probably the correct way to write high reliability code.
Depends on the region. MISRA is widely adopted, and then there are the US MIL standards, ECSS for european aerospace stuff, do-178C for aviation..
coding standard is a part of the story. mainly it comes down to level of rigor and documenting process and outcomes for audit ability. DO-178c