I'm going to offer up this observation, from an outside third party who doesn't know you or yours. Reading through the OP's posts:
Claim the first: There are issues at my job because my boss is playing favorites with respect to responsibilities, we were suspicious this was the case but then he married the "favorited" individual proving there were shenanigans.
Claim the second: As this will be an ongoing issue, sorting it out through HR was pursued. HR seemed to indicate this was the new status quo. Leaving the job is now the only option.
Claim the third: Offending boss is leaving, taking the "favorited" individual with him. Nothing has improved.
A reasonable theory would be to suggest that you were already unhappy with the status quo before this whole relationship was even a thing.
You don't like that job.
I'd put out a corollary to that theory, that your issue with two other people finding happiness in a situation in which you were not happy caused you to lash out and seek to make miserable those people. Maybe that isn't what is going on, but you should really look inside yourself and determine if anything having to do with their relationship materially affected you. At this point, you sound like all of the whining busybodies at every workplace who spend so much time making sure they're being treated fairly they forget to be awesome, easy, employees.
Being a pain in the ass, and being treated like a pain in the ass, is entirely your fault.
But maybe that isn't what is going on. Maybe this is a lecherous old man preying on a young impressionable woman, as many people reading have assumed and I would surmise your lack of detail intended to create that impression. It might not be a mid 30's boss meeting a mid 20's young professional and developing a relationship. Or a mid 40's with a mid 30's. Maybe this relationship violates the "creepy" rule (half your age plus seven; over 60 anything you can get).
From reading what you wrote, and from my perspective of always wanting to see the other side of the story, I'm betting the boss is rolling his eyes. In truth, no part of his decisions regarding this person considered you. None. And while you might think that's a problem, he was under absolutely no obligation to consider you.
You probably think that all good assignment should go to the best person for the job. You've probably never been a boss and realized that when you give new people only shit, you lose lots of new people. You probably think that seniority and superior skills should dominate every decision. You've probably never been a boss who had a senior person with superior skills up and quit (like you were planning to do) over some perceived office drama.
You were probably going to leave anyway, your boss left before you could ragequit over the drama. Don't change your plans, but look inside yourself and really make peace that other people's business is none of yours. You being miserable does not make the happiness of others wrong. You being happy does not make the misery of others wrong. You are objectively living your own life, and nobody but you is responsible for your happiness. Other people arranging their life in awesome ways does not materially affect you. Working with a girlfriend/spouse sounds like hell to me. I'd be concerned about the well being of my boss, not worried he'd treat me like crap. I can always find another job.
You can communicate the effect others' decisions have on you, and hope they make different decisions. But you shouldn't have the feeling of entitlement that the world will adjust to what would be best for you.
I'm happy for your boss and the intern. I hope they have lots of sex and babies.
I hope you find it in yourself to forgive them and forgive yourself.
If two people are attracted to each other and pursue those feelings, nobody they work with is entitled to anything. Workplace policies exist not to stop the relationships from happening, but to give companies the legal cover they need to fire employees if it causes an actual problem.
So for that to be the case here, there needed to be actual qualified candidates who applied for the open position and then were not hired.
There needed to be an actual deficiency in the ability of the intern to perform the actual job tasks assigned.
There needed to be an actual change in the existing distribution of work to favor the intern. I.E., if you were overworked before and are still overworked, you are placing blame in the wrong spot. It turns out your expectation that a new hire would reduce your workload was the problem, not your boss' peni.
The fact that your boss is leaving is proof that the relationship was not just about power, about him taking advantage. The "wife-tern" going with him is proof that she wasn't just climbing the cylindrical corporate ladder. To me, your boss leaving tears the veil off your presentation of yourself as the victim, revealing you to be a small, insecure individual. Only you can fix that.
What I don't see is that you and your coworkers embraced the new hire, gave appropriate tasks and helped them complete, trained them to do the work you needed done, and promptly brought to the attention of superiors when things weren't working exactly right.
What I do see is a lot of bullshit and innuendo. Entitlement, selfishness, and ego.
But again, I don't know you and yours. This might be a case of you work for a scumbag. It is possible. I've just met hundreds of people with the story as put forth by the OP, but it's never been what was actually going on.
