Ha! Wordiness is a-ok.
I interpret "secondary relationship" to mean "we care for and trust each other and we are intimate and we consider each other carefully when decisions are being made, but in the end we are not each other's mutual top priorities in life" and "primary relationship" to mean "we have all that other good stuff plus we are actively building a life together and putting our partnership before all others (barring any co-primaries)". In theory that doesn't have to mean living under the same roof, though it certainly would make it easier.
If you know you are monogamous(ish) and want Mac to be your forever-love with no other main squeeze to support you, then unless you're the sort of person who can happily get by with relatively limited involvement (which it doesn't sound like) then I think figuring out how a co-primary relationship with Lia could look is the only way this can work in the long-term.
Now of course getting to that level with someone doesn't happen overnight... I'm not saying you and Mac need to be primary now. But you can build a strong base for it to be possible in the future, ya dig? A big part of that will *have* to be learning to share with Lia... really share, and value her happiness and her relationship with Mac and support it. How can she ever be comfortable with sharing more of him with you when she knows you're not really on "Team Mac/Lia" and that you would gladly take him almost completely away from her if given the chance?
It's going to take a major shift in thinking and I'm sure it won't happen all at once by any means. But love can lead you to things you never would have imagined yourself being happy to do (if you'd told me three years ago that by now I'd be all "changing the diapers of a child that's not mine, YES, my favorite thing!" I would have looked at you reeeally funny) and, with time and genuine empathy for their love (they call it "compersion"), maybe sharing Mac can become one of those things for you. Otherwise I truly don't see how this can work. Re-building your friendship with her and spending more trio time are great places to start. Slow and easy.
Also, good point about how poly can lengthen NRE, I hadn't quite thought of it like that but it makes total sense.