I'm not really a fort guy, they're very expensive and once the war has moven out they become almost useless. Maybe if you want to upgrade them to level 3 or 4 that could be better, because going for level 10 forts is not worth the effort imo. But yes, you can try building forts in these provinces you consider weakspots. But then again, forts + rivers are extremely overpowered sometimes
I haven't played that mod yet so I can't have on word on your war because I don't know how good each army is. The Russians are a pain in the ass no matter what mod anyways, you should set one goal against the Russians, will be it pushing them away from your borders?, capturing a city like, say, Kiev?, the whole of Russia maybe?. Beating Russia will be a pain in the ass, hard battles, terrible logistics.
Engineers are ofcourse ideal for river-crossing and fort-busting, but I haven't tried a MAR-MAR-ENG configuration yet. In my games, I always build 4 or 5 Marine divisions (mind you, most of my games are as Germany) but they end up acting as garrisons in Norway and Denmark because I never launch Seelowe. Still, if they're not on their landing roles, they behave like some sort of makeshift infantry division, it's quite weak from my experience, just like using Paratroopers as regular infantry.
IIRC Mountaneers lose almost all of their terrain bonuses if they have artillery attached to them, maybe you'd like to use two Mountaineer Brigades + Engineer brigade, you should keep your Artillery brigades for Infantry.