First, the most natural is "go see the film" (remove 'to') 1. If Lorna will not go see the film with me, I will just go on my own. This is ok, as Brett says, it sounds very deliberate and harsh (Lorna refuses to go). 2. If Lorna does not go see the film with me, I will just go on my own. This is more natural than #1 However, I would not say either. I might say: If Lorna doesn't want to go see the film with me, I will just go on my own If Lorna doesn't come see the film, I will just go on my own 'Come' is more appropriate than 'go', because 'come see the film' implies with me or with me + other people

Honestly, either sounds fine to me. The difference is quite subtle. To me, using "will not go" suggests that Lorna has made a deliberate choice, whereas saying "does not go" is more neutral.

Both are fine

If Laura doesnt come to see film with me, I will go myself