TBS,
You and your story have been on my mind since your original post.
I totally understand you being in limbo still, it can be very hard to get over infidelity, and I know the impact that it had on me.
What I would say about this new development is that you and your wife are going to have to do a lot of deep talking about it, and explore how she is going to handle being the boss of the guy she had the ONS with. I know that sounds obvious, but there are loads of issues that she is going to have to get straight in her mind if she is going to be his boss.
1.) Has the guy told others at work about the ONS? If he has, how does your wife thinks she will cope with that when she has to act as his boss?
2.) Is she absolutely sure she has no feelings towards the guy? She said that she felt nothing for him when the ONS happened, but she was talked into it, so unless she wants to build on that, she has to be 1000% sure she feels no attraction for him now. I know she says she despises him, and I am sure she does because of what he got her to do, but the fact remains that he was able to get her to do it. Both of you need to be totally sure of what her emotional state is here, because people can sometimes kid themselves about stuff. She really, truly, needs to be honest about every last part of this, for you, and for herself.
3.) What will you do if there is some kind of conference or other event that they have to travel to together, and maybe stay overnight?
4.) What will happen if she has to 'work late'? Many managerial jobs can require that, but given the new situation, how are you going to handle that?
In terms of coping tools, I would recommend that the first thing you do is congratulate her on the promotion, and for working hard enough that it got noticed and rewarded. That will mean a lot to her, and maintaining the bond between you is more important than ever now. I know you are struggling with all of this, but treating this like a deliberate thing on her part, or letting it drive you apart, is in nobody's best interest.
Then, as I say, you need to talk through the issues I have mentioned, and any others that spring to mind.
If she can have him transferred once she is in the new role, she should do it. However, she - and you - need to figure out what you will do if he decides to cause trouble and go public about the ONS and say that is why he is being moved. It could be that he will move on without a peep, and the new situation with her being his boss may not be too comfortable for him either, but you and your wife need to talk through the various scenarios that could happen.
I think you said in earlier posts that your wife always tells you about any contact she has with the guy, and obviously, that will be paramount now. I think that your wife could get herself a VAR (voice activated recorder), and try to record any conversations she has with him in her office, just for the sake of safety. Maybe this sounds like a cheap TV soap-opera, but the guy is obviously a POS, so your wife should prepare herself (and this is only an extreme possibility) for him trying to pull her to one side and tell her that she better be good to him or he will tell everyone about the ONS. That really is an extreme and unlikely scenario, but you and your wife should talk it through, and figure out together what to do if something like that ever happened.
I know that your job is some distance from where your wife works, but I would recommend that you both start texting during the day, and maybe having a daily catch-up phone call at a point that is convenient for both of you. That will help give you peace of mind, and it keeps re-inforcing the bond between you. And it will help her too, because I am sure she is not comfortable with having to manage that turd.
I think that your wife should make it a priority to never be alone with that guy in any situation, even if she has to engineer good reasons for other people to be around. The man is a sh*t, and he should not ever be in a position where he could try and blackmail your wife.
As I have said, if your wife can get this crumb moved, she should do it, and do it as soon as possible. She should also be careful to keep all of her exchanges with him as brief and professional as possible, and if she has any formal meetings or reviews with him, she should record them, and bear in mind that he may also could record any exchanges. Again, that's a bit extreme, but if your wife decides to transfer the guy, and they have a one-to-one meeting, he may go out of his way to bring up the ONS and play the victim. If you wife 'cracks' and starts saying stuff like, "Dave, you know why I have to transfer you, we never should have done...", and the guy has a VAR recording it, your wife and the company might wind up getting sued for unfair treatment or whatever. So your wife absolutely has to train herself to keep their exchanges 100% 'cold', limited to the basics, and professional.
On that score, your wife needs to think through what she would do and say if the scumbag ever raises the issue of the ONS with her. In situations like that, I have relied on a technique I read about years ago, called 'The broken record'. Basically, you work out a stock reply that you will give, and you stick to it, repeating it like a part of a scratched record that plays over and over again. You can work on this reply with her, but it should be short, 'closed', and to the point. It can be something as simple as:
"I am your manager, and I will only discuss work issues with you. I hope you will respect that".
Repeat that a few times and the message will soon get across.
On the positive side, this is not really that different to her working at that place up until now, is it? She probably sees the guy around, but has no contact with him. Now, as his manager, she may have a bit more contact with him, but as long as she keeps everything ice cold and professional, she should be fine.
As I have said already, she must never, ever engage with him about the ONS, because he may have a VAR himself, and he might try and make a case about unfair treatment.
Again, I've already said it, but I think the pair of you should be in regular contact during the day, a few times, to maintain a level of reassurance and support for both of you. This won;t be easy for either of you, but if you view it as a team effort, a joint project, you will both get through it, and in the process, it may help to bring you closer together. I really hope that happens, but I'm not telling you what to do because I know that grates on you.
Bottom line, she worked with him before, and they were cold and distant. She needs to maintain that temperature and space, without being out-and-out nasty to the guy to the point where he could take action, and things can work out fine. And your wife will move after a year in that post anyway, so there is light at the end of the tunnel if she cannot get that creep moved immediately.
I guess the ultimate coping tool would be one that sounds like it comes from a Hallmark card, but just love one another, talk, and support each other.
Sending you friendship and strength, TBS.