No. A Java string is a "pointer" to an array of 16-bit integers (each element is a 2-byte character). A C string is a pointer to an array of 8-bit integers.
You have to first convert the Java string to UTF8, then allocate an array of 1-byte unsigned integers, then copy the UTF8 into it, and only then can you pass it to a C function that expects a string.
[1] https://docs.oracle.com/javase/10/docs/specs/jni/types.html#...
Probably not; the other fields in the string will be different (the length field might be unsigned in Qt while it's almost certainly signed in Java. Java strings may have other fields that are not present in the Qt string (and vice versa).
It won't be a problem if the string being passed from Java to C is const. It will be if the C code increases the size of the string enough to set the highest bit. Then Java will be looking at negative length strings.