Nesting vs solo
Okay, here is why I think nesting with a partner and solo polyamory are incompatible, from my experience and some unscientific surveying of other people. TL;DR version: when you nest with a partner, the default becomes "us" time, vs all of your time being automatically your own, and your needing to make plans in order to have time together.
More on this (it got looooong sorry):
I *would* believe it if people told me they are nesting and do *not* operate this way - I think it could be *possible* - but thinking back on my experience living with a partner, by the time I started dating a second person and went back to school 5 years into our relationship - well, *that* was when I realized that my partner and I had slipped into a shared assumption that all of "my free time" would automatically be spent with my partner, barring other commitments. Granted, my other commitments had basically all fallen away by then. I had no local friends. I had no volunteer gig. My partner even resented it when I worked on some weekend days. All our hobbies were shared hobbies. It was *not* a healthy relationship. But rather than being some anomalous outlier, I think this relationship was, in many ways, just an exaggerated (to an unhealthy degree) version of couplehood.
Even my very autonomous datefriend - I asked how often she plans dates with her primary, and she said mostly they just see each other because they live together. They do go on dates, but it is not the main time they spend together.
I also asked about this in a general polyamory group on facebook - something like, "Is your free time default spent with your partner?" and "When did this start happening in the course of your relationship?"
Most people said this was the case, and it began when they moved in together. So that seems like a pretty clear indicator that, yes, space-sharing encourages this kind of time-sharing. Logistical entanglement can lead to this temporal entanglement. And this kind of temporal entanglement can lead to ENTITLEMENT - because when you do start dating someone else, or get back into your old hobby, or go back to school for a degree, your partner will feel like something is being taken away - that "us" time is being "taken away" from them, when really it was YOUR time all along. I never want to get into that place of confusion again. At that point 5 years into my primary relationship, when I realized that I could not manage my time at all, I proposed to my then-primary that we switch the default to having all my time being MINE, and I would make plans for time with my partner (and time for homework, classes, my new date, etc). He basically just refused to discuss this. It just never went anywhere. Good thing I left or there is no way I would be so close to finishing my degree. And that is why I don't think you can nest and be solo. You'll be fighting gravity the whole way. Maybe it is possible. But damn hard. Not only does it seem like an easy, comfortable default to slip into logistically, but the weight of society and all social norms around couplehood and monogamy are pulling you into that kind of entanglement.