***Possible triggers, so be ye warned***
I think when it comes to thoughts of suicide, in respect to a Christian lifestyle, it really depends on the motivation behind those thoughts. If the motivation is looking for an escape from the pain and anguish caused by mental health issues. I personally see this as an a understandable reaction to a desperate situation and in turn not particularly sinful in its self. Acting on them is something completely different, successful or not, causes a lot of damage to our loved ones. If the motivation is a form of revenge or to hurt by proxy, this is much harder to reconcile, but, at the end of the day, the Christian faith also teaches forgiveness, so it's not really our place to judge.
As for your second question about how can a supposedly loving God allow so much suffering. I think of the story of Job, in particular, the wager between God and the devil, and how strange it seems that God would even accept the bet. Did God accept the wager because of Job's faith in him, or perhaps, his faith in Job?