I definitely think Auron had his suspicions about Yevons activities, but this was after he came to being Yuna's guardian; he obviously knew that they were corrupted in some sort, but knew making an outburst wouldn't be the right time to do so. He played along as long as he could until he reached Yunalesca, that's when he sought the feeling to let everyone in the party know the truth, even about himself.

The left arm sling is like Renmiri has said, it was never damaged and I get the feeling that once a person has died, and if they are brought back to life, all their pains and aches are cleansed from them; Auron probably decided that he wanted himself to age in appearance, unlike Seymour and the elderly Maester, he wanted it to look believable that he had once been on a treacherous journey, that's why he has the scars and such.