Pointer to one before first element of array
(1) Is it legal to write --p?
It's "legal" as in the C syntax allows it, but it invokes undefined behavior. For the purpose of finding the relevant section in the standard, --p is equivalent to p = p - 1 (except p is only evaluated once). Then:
C17 6.5.6/8
If both the pointer operand and the result point to elements of the same array object, or one past the last element of the array object, the evaluation shall not produce an overflow; otherwise, the behavior is undefined.
The evaluation invokes undefined behavior, meaning it doesn't matter if you de-reference the pointer or not - you already invoked undefined behavior.
Furthermore:
C17 6.5.6/9:
When two pointers are subtracted, both shall point to elements of the same array object, or one past the last element of the array object;
If your code violates a "shall" in the ISO standard, it invokes undefined behavior.
(2) Is it legal to write p-1 in an expression?
Same as (1), undefined behavior.
As for examples of how this could cause problems in practice: imagine that the array is placed at the very beginning of a valid memory page. When you decrement outside that page, there could be a hardware exception or a pointer trap representation. This isn't a completely unlikely scenario for microcontrollers, particularly when they are using segmented memory maps.