Kudos to Sarmatian for a solid post.

It is also silly to think that Russia doesn't mind Montenegro joining NATO. And by now we know what Russia can do if it wants to prevent a country from joining NATO.
Let's recall the process of ways, means, and ends. Montenegro accession is a nice symbolic boost for America, but a very limited one. Also from a military perspective, Montenegro is a tiny country with a not-very-strategic position, and fewer resources and smaller population than the Baltic countries. Its military, though participating in the process (and having participated in various NATO programs in past years) is never going to amount to more than the gendarmerie of a small town. Russia, to voice its displeasure, has more tools closer at hand, in Syria, in Ukraine, Romania, Poland, the Balkans, Scandinavia, etc. for causing disruption, calling attention to itself, changing global perceptions...

It is deeply unlikely they ever had the resources for a coup in Montenegro, or that they could develop them within a year. Slightly likelier would be that they sponsored a disruptive 'suicide coup', but its hard to see how it furthers Russian interests in any respect, only costing Putin some of his hard-earned aura of canniness and further steeling European countries to guard against Russian machinations.

It's a bad idea to try to directly compare this to the Turkish coup attempt in the summer, so I'll simply urge a consideration of how much Russia could have had to play in that affair going by this attitude. It's not out of the realm of possibility, but it assumes so much in terms of unknown or unknowable secret resources, secret goals, secret actors, secret decisions, and secret communications that it verges on 'Aliens did it' for solid ground. That kind of speculation isn't worth the time unless you have something concrete to point out.