nothing will change him if you catch him in this lie and have proof to show him. nothing changed when you caught him in previous lies. so i dont know why you think having evidence is going to result in a miraculous epiphany. it wont.
also, stop having kids. it's obviously detrimental to your health. it's also ridiculous to keep tying yourself to a man who will keep seeing other women and whom you couldnt trust if your life depended on it.
anyways, there's nothing you can do to make him change no matter what proof you have, no matter what you say, no matter how many children you have with him, no matter what investigations you conduct. why? because you cant make someone love you. it's as simple as that.
so stop doing all this crazy stuff as if that's going to change anything. it's not. it hasnt in the past. it wont in the present. it wont in the future.
i dont know why you want to keep all this drama in your life. you might consider therapy for yourself. also get STD testing.
anyways, instead of driving yourself crazy, do one of two things and stick to it forever: stay and be happy no matter what he does (because nobody has the power to change him, except him and he's just fine being as he is) or leave and move on. no more kids. no more sleuthing. no more thinking you can get something on him to want to change his ways. no more hoping, wishing, planning, strategizing, etc. just leave and move on. let him go completely, because you obviously cant handle his manipulations.
so chose one of these two options, because what you need most is a healthy emotional and mental mindset and environment.
also, nobody is a 'fiancee' for 6 years. after all those years and kids, that word just becomes a euphism for "he's been stringing you along because you didnt have the balls to leave him."