Hello there. You will be traveling on a small regional jet operated by one of Delta’s regional partners NOT Delta Air Lines aircraft or personnel.
I want to give a disclaimer that without knowing which specific aircraft you will be traveling on it is hard to say as there is a cadre of different seating configurations on the Delta Connection CRJ-900 fleet as there are different operating carriers (Endeavor, GoJet, SkyWest, etc). That said I believe the arm rests between both seats should raise on all plus the aisle armrest for ADA accessibility. The armrest between seats should raise and lower without any action but the aisle armrest has a release pin underneath that must be depressed to unlock the swinging motion.
I would suggest checking the Delta SkyMiles forum on FlyerTalk as that is mostly road warriors and frequent fliers extremely familiar with the seating configurations whereas this forum is primarily airline employees looking to discuss labor relations and industry affairs.
Josh