Your dad is a 10-month survivor -- that is positive. I lost my dad when I was your age, so I can understand the panic. He died suddenly of heart failure. Your parents really should get their paper work in order. Lucie and I just this past week updated our wills, power of attorney, etc. Unfortunately, people wait under a crisis to do this when all of us should do it while we are healthy.
Seven months in a lung cancer survivor's life is an eternity. Have you considered moving the wedding to an earlier date? Try to stay positive, even if your parents aren't. They need to be surrounded with positive right now.
We cope with the uncertainty by taking one day at a time. We do not play "what if" with the future, but we do move forward with plans, so Lucie has something to look forward to. It has also helped that she focus on things she is interested in, such as reading, sewing, seeing the family, etc.
My heart goes out to you. I wish you well. Keep us posted. Don