I'd say fairly useful in the Republic and the early empire, but less important as time went on and the roman road network expanded.

You have to understand that milestones are just one of the tools that were available for travelers and the military for navigation. They're competing against itineraries and maps (to a lesser extent) in terms of usefulness for travelers. Milestones collect limited information, and they were initially very popular early on in the development of the road network because they were used by wealthy or important individuals and eventually emperors as monuments to themselves. One should think of milestones as an advertisement, "The 20th mile to Caesara, brought to you by the wonderful and talented Gracchus." Not a real quote. I just used the name Gracchus because Plutarch mentions how he set up a roman milestone (Plutarch Gaius Gracchus, 7 I believe?).

Kai Brodesen's article, as well as Benet Salway's, in Travel and Geography in the Roman Empire don't talk about milestones too much, but they really give you a sense of how much more useful and efficient itineraries were to travelers and the military. Check them out: Colin Adams & Ray Laurence, Travel and Geography in the Roman Empire, Abington: Routledge, 2001.

Your second question is difficult to answer because I cannot recall a specific milestone that would prove the intent of a party to disrupt the navigation of an army. I want to say no, but it's not an outright no. You've inspired me to write an article on this, or perhaps I'll just leave that up to you. I say no because I don't see the opportunity for an enemy to do this. Milestones kinda stopped being set up after the fourth century, and that's probably because the empire stopped expanding. And again, I return to the useful of itineraries and maps. An invader might already have better resources and wouldn't rely on this random monument on a path. He should already know where he is going. This is speculation and something I haven't thought about until you asked the question. Thanks for that. But that's why I say no, but it's not outright because maybe someone could have done in the West. Maybe the West did this as the visigoths stormed in. Maybe. Something amusing to investigate in the future if it hasn't already been investigated.

As for the third part, not sure if intent can be proved on that one. If you look up a few milestones in the CIL (Corpus Inscriptionum Latinarum), you'll find a few that have been repaired because they've been destroyed and a few that have been redone with new inscriptions. I'm sure they must have made sure the position was correct before the repair work was done, but if there was no reason to repair a milestone, there would not have been checks to assess its accuracy.