I think that you made the right decision, to play along with him so you could get him to test. After a date rape like you described, and all the handstands you had to do to get his DNA test, of course you are correct in worrying that the courts will not prosecute effectively. They'll say that "your word against his" thing, and ask why you continued to be nice to him, and so on.
It's only natural that you will now have post-traumatic stress over the whole awful experience, made worse by having to keep it a secret. One thing I would do is write up the situation (or keep copies of this post and the others you wrote), naming his name and his country and anything else you have, just in case sometime in the future he gets arrested for forcing himself on someone else, and it happens that he is on trial for several such situations. You would be able to support some other woman who perhaps the courts would otherwise not believe. (Of course, if he is being deported to a country in which men never get arrested for this kind of behavior, that might never happen. But you simply never know.) Keep the record locked up somewhere, or burn it while hurling curses at him some day, whatever is most cathartic. You went through hell, and lots of women are with you on the decisions you made. You did NOTHING WRONG. It is NOT your fault. You have no reason to be embarrassed. He wronged you, and our world as it is structured also wrongs women in this situation. All you can do is know in your heart that it was wrong and you are not wrong.
This all said about post-traumatic stress, please realize that you are letting your anxiety settle on the paternity test because of the PTS, there really aren't legitimate reasons to feel the test is wrong. The DDC is a big outfit, and it does draw attacks from women who don't like their results (especially in the past, when this kind of testing was new). Probably some of the women were correct, and some of them wrong, in thinking that the test came up with the wrong answer. But a lot of that could have been issues with the chain of custody of the samples, or someone faking the test. In the earlier days when prenatal testing was very new, there were more complaints. The interim lab (the one collecting the samples to send to the DDC) often was unfamiliar with the procedures, or guys would find a way to fake the test (before labs photographed the guy and his ID, which I think they all do now, guys might send a buddy in their place), or something in the chain of custody would go wrong. But this is almost all gone now. Unless you think this a*s*s managed to fake the test, I would trust the test. The worst labs have dropped out of the picture, and those that are left really do have good reputations. (I suppose you are aware that the DDC does all those tests on Maury and other shows? Not that I'm up for those shows, but TV shows have deep enough pockets that they would get sued all the time if they announced someone was the dad and he wasn't. They aren't stupid, they won't have used the DDC for this kind of work if they thought they could be sued for false results.)
My suggestion is that you read up on post-traumatic stress, and its effect on things that the person never thinks will pop up. It comes out in the form of not being able to sleep, not believing a man who says something, etc. etc. and stuff that isn't even so directly obvious as a way to worry about the event that happened. And talk to a therapist if you can about the date rape, and your choices afterward. Do NOT buy anyone trying to tsk-tsk you about not reporting it. It's like when someone gets held up at gunpoint and they hand over their valuables rather than fighting. She did what she had to do in order to live, in a situation in which some people do not come out alive. You made the intelligent choice to get what you needed from this creep, and nobody should shame you for it.