Thank you for the reply and the kind, welcoming attitude.
First and foremost, I must say this: I tend to be gentle. Overly-so, in fact. So gentle that only now, after a year of being together, has my wife gotten me comfortable with the idea of being even remotely dominant in the bedroom. So gentle that I am almost never the first one to initiate anything in a relationship, for fear of coming off as overly-aggressive, rough, manly, impulsive, unstable, pushy, needy, or an asshole.
I was raised by a woman who claims to be a feminist, but seems to be a little bit more on the manhater side of things. Or maybe that's just how I absorbed it and I completely misinterpreted my feminist upbrining. This might explain some of my misconceptions about life, in general; but it definitely explains my timid nature.
So in any case, yeah, I'm not used to feeling combative. This is unusual and new for me. I think it might even be unrelated to this problem in particular, and just a mating season kind of thing. Or (gods forbid) maybe this is my prime and it's all downhill from here. :P
Confidence is something we are working on building, and there's plenty of time before this relationship with her friend will have any opportunity to go anywhere, anyway, so I have time to become accustomed to my varied jealousies and find solutions to them and/or uproot them entirely. For this I am very thankful.
I might also mention that I've only been "dealing with it" for a couple of weeks. This is a relatively new development in life, as I mistakenly expected it never to come up.
I must further clarify, though, that I do not intend to begin another relationship. I've never been able to properly manage one, let alone two. How exhausted that would make me, I cannot even begin to tell you.
I've been spending most of my workday doing research on this topic, since I'm the only one in the office right now, and the phones have been next to dead. Thank the gods for small favours. I needed today (though I would have preferred to spend it home with my wife, talking through this stuff in-person as it comes up).
I think one part of this for me is that I have invited my wife to talk openly about how she feels in this situation, and though she has taken me up on a few small opportunities to make individual comments, any longer serious discussion or requests from her end have yet to happen. I hope it's just because she doesn't feel the need to and/or that she's handling them fine by herself right now, and not because she doesn't feel she can be truly open with me about them. More underlying jealousy and trust issues there; I feel awkward when there's radio silence, even when it's normal. I feel like I have done something to make her not trust me. Knowing myself and my haphazard habits of accidentally hurting people's feelings without realising it, that's a definite possibility.
So to your questions:
How do we manage our primary relationship...?
I don't know if I can answer that. That's a pretty big question. I don't know how we manage it, but I definitely felt like our relationship was pretty indestructible before this situation began stirring. I think the only remaining issues have to do with me overcoming my jealousy and just believing and trusting in our relationship and its solidity. That, and making sure she imparts the knowledge that a secondary relationship is never to break up the primary, unless she and I ever hit that crossroads (which I don't expect we ever will. We worked too hard to build this and grow to give up now, or ever).
I kind-of already touched the second question in that first answer. I don't really know, but I think that one's largely up to her. I will certainly discuss it with her, though. I'd be happy to lose the concept of primary/secondary, because what I've read of them, it is largely either destructive, or it leads to short-term secondaries, neither of which is desirable to me.
How do we structure our time and responsibilities...?
I think we will definitely have to work that out. For now, there's an understanding that she only spends time with a secondary when I'm not available. I will have to work to make sure that if I'm available by surprise I manage not to interrupt her previously-allocated time with the secondary, and also to feel okay with being alone for that time.
What can I do, personally, to strengthen...?
I am not really sure if I can handle an on-again-off-again thing, but I'm sure that once I become comfortable with one, subsequent relationships should be easier to handle, and I'm sure that I'll eventually reach a point where it's altogether okay and I can just be happy.
Thank you for your helpful commentary and the questions you gave me. I will definitely be using these as talking points next time my wife and I have a serious discussion about polyamory and her relationship with her friend from school.