Yet, it happens so often, one would think it was OK. Like the Salisbury poisoning, where the victims were referred to as an 'ex-Russian spy', when he was really a 'Russian ex-spy'. Unfortunately, there are cases, where if one is true to form, it becomes rather a mouthful.

Unfortunately, I believe it is because what they mean to say is he is no longer the French President (as if 'French President' was a single term), and attach 'ex-' in front of that term. Similarly for the 'Russian spy', which apparently is also a single term, rather than two terms.

The correct way to handle it is to use the word "former" instead. "Former French President" parses correctly.

"Ex-" binds tightly to the word in front of it because of the dash, but "former" allows "French President" to be the noun phrase being modified.

Only in your incredibly contrived interpretation of the english language. "Former" would be better here but there is still not any significant ambiguity as to what is meant.

It's clear what they meant to say, but that doesn't make it correct. Just because someone can understand me when I'm speaking broken Russian doesn't mean my grammar is suddenly good.

If it's clear what you're saying, then it is okay to say it. Nobody said anything about formal correctness until now.

Newspaper headlines ought to be formally correct.