...You find me the verse that says suicide is any worse than any other sin.
Oh, and
romans 8:1
1Therefore, there is now no condemnation for those who are in Christ Jesus
So, yes, God will still accept them if they fail at suicide, and, if they were already saved, they'd still go to Heaven even if they killed themselves, I think.
Um, not sure about the last one. I don't think it would stop them being forgiven, but it's still a stupid thing to do. Jesus gave His life for us, so we should be at least willing to live ours for Him.