Has your farrier pressure tested his feet? With the tripping, he could have developed an abscess or some sort, or be in some kind of discomfort from his trim. Depending on the test, he could come back positive for EPM anyways (even if he isn't diseased). EPM appears to make the horse drunk though. If he's not swaggering around like an idiot, I think the EPM test is just a precausion.
I would focus on his feet with all the tripping. You're right though...I'm not sure I would have done the test if the vet told me that 90% of the horses in the barn would have tested positive without being effected. So what good is the test if the horse is positive, not diseased, and you would be treating it for something it really doesn't have without fixing the actual problem?
This could get really frustrating.