Yep that's the standard in embedded on bare metal without memory allocation. There is a mechanism in place to synchronise data during interrupts, so it's not really direct write. Usually also coupled with a two complement variable or similar to make sure memory is not corrupted for safety critical data.

nah, by looking ad other people's firmware it's because most of my embedded colleagues are goats stuck in pre-ANSI C. You can always declare a "global" static, so it's not on the stack or heap, and access that via functions.

Nothing wrong with source-file-level statics, you're bound to use them