I think you need to immediately find a good lawyer who is well versed in circumstances such as yours. Hiding the passport is a good idea, but it may not prevent him from taking her out of the country. If he has diplomatic ties, they probably also have private planes, and they may not even check for passports, especially not one of a child. You have traveled there, so you probably have an idea of what the security is like. Some countries only have a problem letting you in, others with letting you out, and some with both. My husband travels internationally for work and has told me that some places don't even check his passport (which may be because he is a crew member, but still, it makes you wonder).
A few things to consider- since he has been trying to get you to use his Ipad, left it with you while he left on a trip, and left all of his stuff open, he may have actually wanted you to see it. I know it sounds crazy, but people who do what he has been doing would probably not be trying to get you to use their Ipad- he would want to keep all of that very secret and keep you from finding it. He may either be trying to get you to do something crazy (like leave the state with the kids) so he has an excuse to steal them, or maybe he just wants to give you a reason so he can get the divorce he wants. Sorry to shock you with this, but I thought it needed to be mentioned.
That being said, I don't think you should let him know that you used the Ipad or that you found all that stuff. Just try to act the same as usual while you talk to your lawyer. Don't do anything until you've gotten some good legal advice. After that, you may want to consider talking to your husband, telling him you'll grant him a divorce as long as you have custody of the kids (or maybe partial custody-only in the U.S., and only with supervision). If you think marriage counseling may work, then definitely try that, too.
You are in a tough situation, and I hope you have a good church to support you. If you don't, I hope you find one. The support and prayers of my church friends has helped me and my family through a lot of tough things.