Hi sikan - Many of your thoughts resonate with me It will take a long time to come to terms with what has happened. In the meantime, some ideas.
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OW was also there and made a concerted effort to talk to H even though H was not actively encouraging this and was very much at club with me.

Every time that H can see OW as demanding, it is an opportunity for you to shine as the better alternative. Continue to be the fun person, and draw him closer, without demanding attention, which sounds like exactly what you are doing
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I'm afraid that they will just end friendship because of hurt feelings instead of addressing the fact that friendship was inappropriate and because of this it could resume again. Does that make any sense?

Yes it does make sense, but in time most of them do wake up from the alien grip and do make the break for good reasons. Sometimes there are several break-ups. That's why pushing them to end things is not really constructive, it will happen in its own course. What we can do is influence by showing what the alternative is.
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He doesn't have many friends that he sees on a regular basis. OW became a friend to H, second to his R with me. I would love some suggestions as to how to get through this phase of the sitch.

This seems to be a common theme here. Make sure there are new activities - we planned salsa one night a week, I learnt how to skate and roller blade so that it was something we could do together, and we moved house, a project that kept us both entertained and occupied for a few months, which turned out to be critical months in the end of the affair.

Hope this helps. Slowly

A Liberal Allowance of Time