/Can/ one run mainline GNU/Linux on the Cosmo Communicator? On their website, it says Linux and Sailfish OS support are planned, but not ready yet. And, will it be pure Linux, or will it be running on top of Androids HAL (Hardware Abstraction Layer)?

My understanding is that the Pinephone, Pinephone Pro, or Librem 5 are the only pure Linux phones available... and the Librem 5 is 'barely' available given the long lead time on orders.

Technically, Maemo Leste is a pure Linux option on some older handsets, but I'm not sure any of them support VoLTE, so increasingly that would mean no telephony support... depending on where one is in the world and whether they've dropped 3G and 4G non-LTE calling.

I would love to know if there's other pure Linux options.

Last I remember checking in on this properly, it was basically as you are asking, but they also had some thing stated somewhere about supporting things like Kali Linux. So I think the answer is ultimately "yes", but eventually.