First the pictures. If he replied to her photo messages, it often shows up on a bill as a photo message he has sent. Send him one of your own photos to verify this - get him to reply in text format and see if it matches what's on those bills. That said, it's unlikely that these photos were all innocent and he probably sent some of his own that he can recall, but doesn't want to tell you about.
He can remember more details than he's telling you, but like some of the other things you've mentioned, he's minimising and lying. He will swear blind this is not to hurt you, but it's actually self-protection.
Your imagination about what happened between them is likely to be far worse than the reality, so give some thought to whether that knowledge might help you, but he does need to be honest with you.
He needs to start conversations. If he isn't, he's hiding from it and hoping it will get buried.
As regards him changing, yes it's possible but it takes a long time and a will to do it. Counselling on his own would also help with that, although I appreciate it would be expensive. Personally, I think couples often rush to relationship counselling when the relationship wasn't the real problem. Also, if you still see evidence of 'people-pleasing' in him, it will give you no confidence at all.
Did he read anything? Did you?
Based on what you've posted, I think you're feeling the way you do because there are massive gaps in the story, you're not convinced he's capable of change and you're putting too much focus on 'staying together for the children'.