So I'm asking; do I wait for him to mourn his last relationship?

I personally wouldn't be OK with this. I don't want to be in a relationship where I have to wait for someone to decide if they love me more or love someone else more.

It's setting yourself up for pain. Do you want to be around when the day finally comes, and it sounds like it's 50/50, when he decides you're not the one he wants to be with after all? Do you want to go through the pain of not only rejection, but rejection for someone else?

Do I wait for his impending promotion so our relationship isn't against work rules?

That really sounds like the absolute least of your problems. Your relationship not being against the rules will not clear up his feelings between you and another girl.

You think you're hurting now? Imagine 2 months from now when you're laying in bed thinking "I think we're finally going to be together...", and you get the text or call that he's going to rekindle his relationship? And I'm not accusing him of this/saying he's going to - but it's a possibility because of the situation you guys are in (He has feelings for 2 women) - what about the conversation he has with you after he cheats on you with her?

Or am I being a complete masochist who needs to let the whole thing go and find my happiness?

Everyone on /r/relationships says "GET OUT, OMG RUN AWAY!", but in this case - I can't help but agree. You're in a situation where there are just as many bad outcomes as there are good outcomes.

I think at the end of the day everyone should be with someone who wants to be with them as much as they want to be with that person. Everyone deserves that, don't you think you deserve it too?

Find someone who won't make you wait while they decide if they want to be with you.