I don't think we can know, because mortal sin depends on the person's state of mind, which only God knows and He knows us far better than we know ourselves.
What's "ideation"? If it's thoughts of suicide, I'd say such thoughts can be sinful, if you wilfully dwell on them, but probably not mortally sinful unless you start making plans to carry them out. We all have thoughts that pop into our heads, sometimes of sinful things, but since this is not conscious it's not a sin. This is one of the things you should ask a priest - I'm no expert on these things. The depression probably takes away full consent of the will, but it's best not to dwell on these thoughts for both your mental and spiritual help. Good to hear you're getting help, and I'll definitely pray for you.