That’s not what the sentence says. It says that 1) the police believe it to be the case that the body of evidence (implied) they have regarding Person A’s involvement in Crime X is so strong, that 2) it would convict person A, if he were still alive.
They are reporting an opinion held by the police. Of what jounalistic standard does that run afoul?
Fair enough; as I said, I didn’t find it jarring at all, and I had to read it a couple of times to see what anyone would find jarring about it. It required no mental rearrangement for me to understand it.
I spent a few years working as a writing tutor, and so I became very familiar with poorly-worded sentences; it may have resulted in a higher tolerance for them :).
Daniel
No, what you wrote is not what it says. It plainly says they believe they have enough evidence to prosecute. The word convict does not appear, nor is mention made of the strength of the evidence. You’re reading a lot into it that is not there at all. Having enough evidence to prosecute is not the same as getting a conviction (not by a long shot, otherwise there would never be a “not guilty” verdict), and believing there is enough evidence to prosecute is not the same as believing there will be a conviction.
Actually, while Magellan is arguing a quite different question here, he’s hit on the solution to the execrable sentence.
Transform it from
To
That remains slightly awkward owing to the need to convey contrary-to-factness in the infinitive, but
???
Why are you throwing in “successfully” and “convicted?” They don’t affect the clarity or grammatical correctness of the sentence, and they are not part of the original statement.
“Inspector Y stated, this morning, ‘We’d have nailed the wanker, but he died on us.’”
The sentence is grammatically fine.
My only support for that claim is that it does not break any rule of grammar. I’d have to see what rule someone thinks it breaks before I could say any more about htat.
The sentence is ambiguous (which is not a grammar problem), but in almost any conversational context I can think of in which the sentence could be uttered, the ambiguity is easily and immediately resolved by reference to that context.
-FrL-
Convicted is nowhere in the original sentence. It shouldn’t be used, it shouldn’t be implied. Total red herring.
But isn’t part of the evidence that the person to be prosecuted is alive? 
A more common example of such an ambiguity: Suppose I’m at work on the last day before a vacation, and a co-worker says to me “If I don’t see you before then, have a nice trip”. To which I usually reply “And what if you do see me before then? Am I supposed to have a lousy trip in that case?”.
Yes, all of the people I work with already know I’m a smart-alec.
You are right, I got too caught up in the sentence. I stand corrected.
So, I submit: "“If constable X were alive, police believe they would be able to prosecute him for [crime X]”
“…he up and died on us” is much more evocative.
But in this sentence, you lose the information that their belief lies in their already possessing enough evidence.
I think it is safely implied. Why else would they believe they would be able to prosecute him?
What I mean is that you lose the information the police already have the evidence, the alternative being the evidence would have relied on the constable being alive.
But why would they believe such a thing? Obviously because they have some degree of evidence, enough for them to come to such an opinion.