Just wanted to confirm that if you use a placeholder like ##CUSTOMERFIRST## and no first name is available, will the placeholder fall back to nothing?
Yes that’s correct. Of course you still need to be careful, because if you word something that implies a name to be there and it isn’t it could look odd. But it is useful for things like
Your request is being …
Since in the example above if the name isn’t there nothing is lost and it will still be correct.