I don;t think we can state definitively how mom was killed. There is suggestion the kids were strangled, by the defense motion that specifies their necks. But the gunshot/bullet thing could just be standard evidence collection. It could be exactly as you've said, though. Has the affidavit been released yet?
The method of deaths has not yet been formally disclosed by LE
The defense comment about bullets, etc is boilerplate request and does not at all mean that firearms were involved. It's a very normal legal request