As with so much in Faulkner, nothing is easy or simple. There are challenges all the way through with everything and everyone. Consider the warning on the watch that haunts Quentin, to a great extent:
I give you the mausoleum of all hope and desire.
It is in this statement where one can see both external and internal conflicts. There is a desire to want to overcome the external challenges that limit his own sense of self, but there is also the internal that wages battle within Quentin's own mind and soul. This makes his life extremely difficult, where suicide becomes reality, and one that makes the discernment between internal and external challenging. I think that similar to how the narration in the second section flips between real and fantasy, one has to concede that the conflicts that present themselves also move between internal and external. For example, the feelings towards Caddy would be something that operates on both levels. On one hand, there is the act itself which is socially inappropriate, causing the external conflict of Quentin versus society. At the same time, there are the conflicted feelings in which Quentin recognizes his own torn feelings towards the situation. There is a feeling of love and tenderness towards Caddy as well as uncontrolled hatred and violence. It is here where there can be conflict seen in terms of representing internal and external, something that defines and haunts Quentins' character. In the end, I think you can identify the conflicts present and then make a case for them being internal and external, representing qualities of both.