Problem is that you've allowed content modify (w) on that file, which means ANY content, inc none or a complete replacement.
Either you want this or you do not. If not, then no-one should be allowed to (w)rite to it.
You need to think carefully about exactly what you want to achieve; there's no way to say people can only modify a bit of a file, its meaningless.
(More accurately, to do that, you'd have to write your own access program or use eg a DB)