Sorry to say this about your husband but it sounds like he is not too bright, especially when it comes to technology. I agree that you should not let him know what you are aware of. Do however start collecting all evidence you can find. Go to a free service like DropBox and get your own cloud drive. Whenever you see any incrimnating evidence take a screen shot with your ipod and upload the photo to your own private (password protected) drive online. That way if he walks out and takes the iPod you will still have the evidence. If you have a laptop at home be SURE to set up a keystroke logger (mobistealth.com) that way you can monitor all his communication and what sites he is on etc.
Keep all of this to yourself. I have a friend who got too emotional over his divorce and it cost him literally over $300,000. It is an emotional thing to break up, there is no question of that, and my heart goes out to you. However, you are going to need money to help raise your babies. If it comes down to that your attorney will be able to use any evidence you gather before splitting to your advantage. Also, you can share it with the brothers in the hall. If the other girl is a witness you can hang her out to dry as well. Stress to the elders that while you may not agree with everything the WTBTS says you do fully repect the sanctity of God's provision of marriage. Try to keep your emotions under cover so to speak and get the goods on him first.
Whether you stay together or not the best place to be is in a position where you have as much power as possible. I wish you good luck.