I think this is Alma's home town, where she was probably born, and the minister has probably been a family friend for most of her life, as is likely to happen in small towns back then.
His comments were that of a close family friend, any thought of a crush on her are pretty preposterous.
I think the wedding scene was no more than it was intended to be...a small family wedding on a tight budget. I dont think the bloke with the dark hair was placed in the shot on purpose nor were there any hidden meanings or interpretations, hidden in the scene.
It was there to show the next path that Ennis followed after his adventure with Jack on Brokeback Mountain. It showed that life went on as had been planned before Brokeback and that Ennis was committing to a life that was considered the normal life he should be leading.....regardless of what was going on in his head at the time.