I have to second Annabel's thoughts. What she describes is they way it worked out for me and my two men. We were extremely lucky to 'have done the work' that is required to come to a point where 'co-primary-status' is possible without us noticing that we are actually doing it. It took several years.
None of my men would be able to live with something less than 'primary status'. They are both monogamous and don't want to change that part (up to now ^.^). It is possible to be content with a polyamorous relationship even if you have mono-standards. At least that's what they have been able to get across to me. I am a worrywart, I see problems all along the way, but even if I worry about them not being happy, I don't have proof at the moment
The process (being in contact every day, sharing everyday life stuff, worrying/laughing together) lead us to this situation, and when I finally told my husband that there is another man I am in love with (our mutual friend of some years) he was (just like you describe Lia in a way) able to give, and give, and give. BAM – co-primary-status established within some weeks after our relationship started.
It is so precious that you already have a deep friendship you can use as a foundation for the newly developing metamour relationship between you and her. There hasn't been such a deep connection between my two men, therefore I would consider you two lucky
It sounds really promising, good luck to you and your loved ones.
Facts: 30, female, bi, v-type relationship with Sward (husband, straight, mono) and Lin (boyfriend, straight, mono), poly-fi and co-primary.