I think that you are right that our standing before God is based on Christ alone. Passages like Romans 8 tells of God’s great love for his elect so much so that nothing can separate us from him. Therefore, logically, even suicide (as horrible as it is) cannot separate us from God. In fact, this question can be generalised to the question of whether a Christian can be forgiven of any unrepented sins (eg. he steals some money but did not get a chance to repent; or sinned unawares). The answer to which is yes, he is forgiven. Christ’s death surely atones for past and future sins and give us all-time assurance. Again Romans 8 speaks of Christ and his Spirit interceding on our behalf - pleading to God to forgive us. It is comforting that Jesus prays for us even when we do not. But not only so, God the judge is now our father and so will forgive us. The misconceived idea of suicide as the most fatal of sins is probably a result of a misunderstanding of repentance and how people are forgiven.