I can understand that you really want to know where he is living and who he is living with but it will all eventually come out in the wash, it's not necessarily helpful to know it now.
As soon as there is trouble in "paradise", as there will be when OW finally gets to realise that she is with a compulsive liar, he could likely put massive pressure on you to come back "home", you don't necessarily want that to be too soon, you need time to absorb all of this.
I know in your mind that you have been picturing OW as someone who went out to snare him, maybe she is, but in all likelihood he built a web of fantasy and chased her. You know he probably is a serial adulterer from the times you have described in the past when he went all strange on you. He is a coward, you know the script of these cheaters is to come back telling you all about how unreasonable she is when she finds out the real him, trying to make you the OW from his affair partner.
Keep strong, don't worry about him having read your letters, there's nothing there really that changes anything, he will be finding out your solicitor's name soon enough, he can see you are truthful about the pregnancy, he's probably calculated your Dad's possible contribution long ago - he is clearly tight as hell when it comes to money.
You said in one post how can you ever trust again? It will come, it will happen. Next time, the minute that someone treats you without the love and attention that they should, you will be aware of it. One incident of leaving you by the side of the road and you will finish with that man, not make excuses for him or think you should be better. Your radar will be good for meeting an honest man, one who does not want you in his shadow but who is inspired by seeing you develop the potential that all those neighbours and friends have seen in you.