Did you fit the final fabrics on the dress form too?

Pics?

I leave 1" side seams. Is there anything to let out? Even an inch or two will make a difference. The other option would be to add a panel of fabric at the side seams, under the arms. I'm having trouble envisioning the collar situation.

Without pictures we can't help you.

I don't know what level of sewing you are at and I hope it is not insulting to you but, did you allow wearing ease and did you allow a little extra because you were sewing a heavier fabric? Remember that your dress form does not raise it's arms and does not breathe. These two allowances for ease can make a critical lot of difference. Also, if you are seaming two layers as one there will be a thicker seam allowance on the inside and that takes up space in the garment and makes it more rigid.

If it is a silk like fabric, it may be that it has no give, either. It may be woven of fine fibers and very strong and ungiving.

About the only thing that you could do at this point would be to open up the side seams and add a lengthwise piece to the sides. You could make that piece decorative such as a ruched piece. Your neckline may have to be enlarged to make that collar lay flat.

I would assume that the collar is in the tulle overlay? Straps on the underbodice and collar on the outer one? Straps and a collar?

It sounds like the garment is lacking an ease adjustment to the pattern.