spoilers about NATO and Russia campaigns
I don't think we will have Russian support to Transnistria (other than the units already there, of course).
Since the scenario happen in Akkula's story, we know that Ukraine is well locked and in stronger position than it is IRL at the start of the war. In Nato 1-3 missions, the player capture both DPR and LPR ; and in NATO 6 (when WWIII start), the Russian offensive in Ukraine is stopped at the border. It means that air support from Russia to Transnistria would be very hard, since it suppose to fly above more than 500km ennemy territory with NATO air forces and air defenses already in place and ROE that allows to shoot down on sight.
Following NATO missions also shows that the Black Sea theater is also in favour of NATO, that capture both Crimea and Sotchi during the war. In Akkula's story, Russia has better result in the fronts in the North than in the South, and Russia seems to be pretty weak in the battle for the Black Sea. The turning point is only at the end, when Russia manages to capture Kiev.
So, I conclude that for the duration of the war, Transnistria was certainly alone and can't count on nothing more that the Russian units already positioned in their territory.
In NATO side, we just see "local" armies. Either "big" NATO help is arriving later, either they are already very busy against Russia and count on armies there to make the work themselves. After all, the player core is a major NATO formation, and the russian player also face many major NATO assets early on during his campaign. Everyone can be too busy to assist in this secondary front.
Transnistria in itself being very small, it could be argued that a joint Romanian/Moldavian/Ukrainian forces attacking from every directions could be considered enough to capture it with very good chances of success.
Here's my bet : neither this ad-hoc coalition nor Transnistria will have any help during the scenario, unless maybe the few russian units already there or a few air units dispatched to assist "Moldovian coalition".
The scenario ?
Certainly about removing a potential threat on NATO rear during the first months of the war. Since Ukrainian also dispatch units (ant there is war on their borders), I guess that Cobasna depot is a target and a main reason why Ukraine participate. Ammunitions were very badly needed, and its capture would solve a lot of problems for them.
Another option : this scenario happen before 2017's WWIII.
I announced my bet, let's see if I'm right
I'm just a simple, white, old man. 
