Ok, a few thoughts here.
First, I don't think the initial acceptance of the friend request was a big deal. Unless you and TS have some sort of agreement around showing each other requests from the opposite sex or something like that, but it doesn't seem like you do.
Second, I don't get the impression that TS cares to play marriage police. He would rather be notified when something happens than be monitoring you. Which is why it's all the more important that you brought this to his attention before he had to discover it in some innocuous way. You did, good job!
Third, I have to agree with WOES and foreverlabeled, the scrolling to look for OMs name is of far more concern to me than anything else, because of this:
Maybe this friend was as out of touch with OM as he had been with me. It would calm me down considerably if OM wasn't on his friend list. Scroll through a thousand names, wincing every time I see someone we knew in common. Of course, there he is.
You say that it would calm you down if OM wasn't on his friend's list (emphasizing the you here because it's not focusing on how BH would feel, but you). Why? My guess is that this might tie into the conflict avoidance we've discussed in the past. Maybe some small part of you thought that if you didn't see OM's name on the list, that you wouldn't need to bring it up at all and all would be well? The wincing at friends in common seems to support that, as each of those winces indicate that you were hoping the inevitable wasn't true. Why hope that his name wasn't there if that didn't have some sort of more favorable outcome than the alternative? Again, not exactly sure what that more favorable outcome would be, just that there must be one. If his name being on the list was inconsequential, there would have been no wincing at all.
Truly, I was looking to make sure he wasn't there, I was practically praying that he wasn't, but the need to know is in itself disturbing to me. It gave him an extra level of attention and relevance when I should have just turned and fled.
Seems like you're understanding the potential this has to be problematic, which is good.
He doesn't get angry that I clicked accept without thinking, he's glad I unfriended my way back out of the situation, he's really glad that the idea of hiding anything from him was unendurable.
I don't see anything in this list about his reaction towards your searching of OM's name, so I have a possibly uncomfortable question to ask - did you tell TS that in between your accepting and unaccepting, there was a period of time when you scrolled to look for OMs name? Or did you just paint it as "I accepted this friend request, but then I realized that it could potentially put me in contact, even peripherally, with OM, so I unfriended him, and I just had to come and tell you!" I'm pretty sure you did tell him because you know he'll just read it here, but I had to ask.
Look, nobody's perfect, and you handled it pretty well considering the circumstances, at least in terms of disclosing it to your H. In a perfect world, I would have left the page up without doing the unfriending, woken H up, let him know what had happened, then let him decide how to go about it. Because I would want him to be able to verify that there were no untoward messages exchanged between this old friend and I before I had unfriended. That's just me though, it sounds like he was happy that you had already taken care of it, and you know him better than anyone.
The more I write about this, the more I think that the advantages of blocking OM outweigh the hypothetical disadvantage of him noticing that I did it.
THIS! re: blocking, I think it might be helpful for you and TS to revisit this. I get that back when you guys first discussed this, he didn't want OM to see the blocking as you giving him some sort of attention. But a lot of time has passed, and I think you are far enough out where you could take another level-headed look at this.
Blocking literally makes it look like the other person doesn't exist. You would not show up to him, and he would not show up to you. I had to do this with an ex boyfriend who was stalking me, as well as the "friend" he cheated on me with. Given that blocking was new to me, I ran several different tests with various friends to see how far reaching the block was and discovered it is literally as if you do not have a Facebook at all. If they search for your name, your profile will not show up, not even as a little thumbnail, nothing. If you had OM blocked and you were to comment on this mutual friend's page, the OM would not even be able to see your comment at all, nor would you be able to see any of OM's comments. Now this was back when Facebook was still in its early days (think, only college students) but I believe that is still how their privacy function works.
It might actually give TS, and you, peace of mind knowing that it wouldn't even be possible for him to pop up at all. No possibility of him "sliding into your DMs" or any of those other accidental pop-ups that you fear might trigger TS.
It's good to know that my internal compass reacted violently to anything that felt like deception.
I think it's awesome to see how how bodies can keep the score in a good way - by letting us know that doing something that works against our own authenticity or self interests (even if we "want" it to be OK) will prompt a physical reaction.
Yes, this is good! Just remember, as I know you already know, that many a person has started down the wayward path of deception due to a desire to push away that very same pit in their stomach. Two paths diverged in a wood... keep taking the one less traveled by.