The point of this exercise is to see if I can do normal Linux things on a UT device vs. let's say using PostmarketOS. This is a valid comparison and an often made one between the OS options.
I think this is a poor distinction to make. Ubuntu Touch is not a traditional Linux distribution, so no, you cannot treat it like one. It is an OS designed with phone/tablet use in mind, and with security restraints to that effect. It's only a valid comparison for people who do not want a phone or tablet, but instead want something they can treat like a traditional Linux distro, in their pocket. And those are two very different things, for which making the comparison in such a manner, doesn't promote the benefits of such a system, and treats the traditional and less secure way as something inherently better.
Partly this ability to play with this stuff is why we're on Linux. Partly it's also because of privacy advantages.
Android is still Linux. So far, the things you've talked about in terms of building and running an app directly on the phone, have absolutely nothing to do with whether there is a Linux kernel running on the device though. Linux is running on billions of devices in the wild. Simply because you can't ssh into those devices arbitrarily and do some traditional things you might be used to on Debian or Ubuntu on your PC, doesn't make those devices not be Linux. Android is the same.
In any case, I've concluded that Libertine isn't the answer but need more space on the host to do more experimentation. Hopefully the Pinephone will give that space.
Libertine is the answer, as it is what provides a container for installing and running legacy apps. However, it doesn't magically make those apps responsive or usable on phones, nor does it enable a full traditional Linux distribution experience. Likewise, Pinephone is not an answer here, as it is not the only device that UT will be supporting. The rootfs contents (in terms of what is supported by UT) must remain consistent across devices, regardless of whether they use libhybris or not, and whether or not they are stuck with Android style partitioning. Doing anything different in this area across different devices will only make it harder to support, harder to build apps for UT, and harder for users to use UT. Those are all things we clearly do not want.