Sounds like an interesting situation you've fallen into. You've read the Ethical Slut, so the concept of scripts shouldn't be news. It sounds like your response to get protective about your relationship with Mac, and desire to compete for the minivan role are part of those scripts...the same ones probably from spending a lot of time being married.
Could it be that running these scripts is part of what's causing the friction between you and Lia? Perhaps you could give some thought to breaking away from the scripts, and find your connection with her. You may not have planned on or chosen poly, but since you're here, might as well see what you can do. But part of that will be doing what you can to avoid turning your relationship with Mac into a zero sum game between you and Lia.
Welcome to the Forum.