Well, like he's doing now in Eastern Ukraine. And I never said that there was no local support, after all he's also trying to get local support in the Baltic states (Like that speech by Russian officials a few days ago).
I know what Article 5 say, thank you. The fact is that 'armed attack' is, as far as I know, not clearly defined anywhere. However, an armed attack on a country's vessels inside Europe is clearly stated to be an armed attack.
And what make you think it won't work? What does Putin has to loose by being increasingly aggressive with the Baltics and trying to stir up trouble there? In the worst case, if the costs seems to high, he can just stop.
The things he is doing now in ukraine can not be possible in baltic states, especially in nato/eu states. To get support for russia minority you need first a big trigger. Something like what happened in ukraine when the pro russian president was thrown down, so a lot of rusian in ukraine where not happy with that, as a lot of them voted for him. But that is also not enough as russia will need to actually send troops there in some form, and that will be impossible as the country is in nato/eu.
There is no clear definition what armed attack is (well there is in a way
https://ruwanthikagunaratne.wordpress.com/tag/armed-attack/), but article 5 is implemented for countries to act on things where they are threaten from real attacks. Taking a boat and releasing it is not one of them as I said, as same as taking a potential spy near the border. Article 5 is for direct and indirect attacks from military/terrorist organization, and that was the point of it implementation after USA was attacked from terrorists. Saying that imprisonment of a boat is attack on nato (as you say they can use article 5 for it)is simply incorrect.
The thing that makes me think it will not work are several: The countries are in EU/NATO (only that will make it not work). I can tell you more, russia direct acting like that would be direct attack on EU and nato, and russia will lose a lot of allies and will be hit by real sanction. So putin has a lot to lose, and almost nothing to get from it, unlike what you say here.
While russia is extremely powerful in its military, and is more powerful to most of countries 1 on 1 (after all it's in top 5 military powers in the world), it can not compare again EU. But it is very stupid to even talk about it, as much as talking about USA attacking russia/china.