My 2 cents: It probably can't hurt to improve shoulder stability and upper back strength. From what I've read, slumped shoulders often aggravate these issues.

Since you can't raise your arms, you can't do overhead press. And you can't do pull ups or chin ups. (Yes, women can do both with practice.. . . ) So the best exercises are ruled out. This is frustrating.

If you can, try inverted rows, reverse flys, one arm dumbell rows, and deadlifts.

Inverted rows admit for some progression. With your feet elevated, they can be made more challenging, when you get to that point.

You can lay on an include bench for reverse flys.

For the exercises that can be weighted, try to add a little weight every workout or so. Keep the rep ranges low (5 for deadlifts, <12 for flys, and <15 for rows). Work on strength. High reps with 2 lb dumbells won't do anything useful. You'll get tired, but not strong. Don't waste your time. Strength is key here. You must lift heavy. And you can over time with gradual progression.

If you can squat, skip exercises designed to strengthen you core. Do heavy squats (with a barbell). Start very light and gradually increase the weight doing 3 sets of 5. You'll do more for your core than endless hours of planks and crunches. You simply can't do a heavy squat without a strong core. Your entire body will get stronger as you make progress. . . .