Putting that aside, even the official parties to the war are allowed through the straits to return their ships to their home ports (unless Turkey is directly involved in the war, in which case Turkey can block all enemy vessels, even those returning to their home port). Since the only coastline and naval bases of Bulgaria and Romania are on the Black Sea, that rule covers their entire navies. It only really disadvantages Russia, since Russia is the only Black Sea state (other than Turkey) with naval bases outside of the Black Sea. It means Russia can't send reinforcements to the Black Sea from their Pacific, Baltic or Arctic fleets.
In a scenario in which Romania or Bulgaria actively joined the war, it would be a direct Russia-NATO war, including Turkey. In such a dire scenario, Turkey would likely announce the unilateral suspension of the Montreux Convention for the duration of the war, and allow NATO warships unlimited passage. Russia would consider such a unilateral suspension of the Convention to be an act of war, but in such a scenario Turkey and Russia would already be at war anyway.
> Vessels of war belonging to belligerent Powers shall not, however, pass through the Straits except in cases arising out of the application of Article 25 of the present Convention [no longer applicable] , and in cases of assistance rendered to a State victim of aggression in virtue of a treaty of mutual assistance binding Turkey, concluded within the framework of the Covenant of the League of Nations, and registered and published in accordance with the provisions of Article 18 of the Covenant. https://timinhonolulu.files.wordpress.com/2022/02/montreaux-...
So basically, if (a) Turkey declares that there's a war and (b) Turkey is not a party to that war, then (c) Turkey is authorized to close the straits to warships of belligerent parties.
There's some exceptions in there about ships returning to their base, but it'd be a pretty big loss of face for the Russians to beg Turkey to agree that a sudden rebasing of ships qualified.
I'm Romanian, I live in Romania (Bucharest), so as I'm geographically close to this I've been following this particular Danube issue closer than the others, but the thing is that the situation on the ground is still pretty muddled. For example, just as the war started the Romanian authorities had suspended commercial navigation on the Chilia Danube Delta Branch, the one on which the Romanian-Ukrainian border is situated, for fear of Russian bombs. Presumably the navigation has been allowed to resume, but nothing official has been made public in the Romanian mainstream press, the only related news was something about the Ukrainian authorities being unhappy with us (Romanians) asking for navigation-related customs taxes that were seen as too high, so I assumed that commercial navigation had, in fact, resumed.
There was also our prime-minister meeting with the Bulgarian side (with which we share hundreds of kms of the Danube) and making a statement afterwards about how the Danube should be made easier to navigate (i.e. a strong hint made to the European Union to send us money to do that). If that indeed were to happen (i.e. if the Danube would be made easier to navigate) then the Ukrainians would be able to send a large part of their exports to Central and even Western Europe (via the Danube-Mainz-Rhine canal) via fluvial ships, at a fraction of the cost they now have to pay doing that using rail and road-transport. That's why (among other things) the Russians have set their eyes so strongly on the Budjak region (i.e. the Ukrainian region just North of the Danube's Delta) and that's why they have bombarded this bridge [2] that leads to that region at least three times in the last few weeks.
[1] https://en.wikipedia.org/wiki/International_Commission_for_t...
[2] https://www.google.com/maps/place/Pidyomnyy+Mist/@46.0749075...