I fear all of this is dancing around the nasty core of the problem: generic writing to C style strings can't be done without extra information. You can't write stuff to memory without negotiating how much room is needed and available, and optionally moving the string. Silent truncation will cause bugs. Buffer overflow even more.

Fixing this now is hard: Writing to 0-ended strings require manually tracking lengths. Expanding a string without allowing malloc is misery.

The only way out I see is basically starting from zero: ISO C should define an API with a (pointer,current length, max length) struct at its core, pointer pointing to a 0-terminated C string. You can read it, but changing it requires using functions that can error out and/or malloc more memory. There are already multiple libs like this, but C has none. If the struct would be ABI, non-C programming languages can pass strings between them.