It's my understanding that PureOS is currently Gnome based and that they're heavily leaning towards that remaining the default, but that they haven't made a final decision yet. It does sound like users will have the option to install either though.
We certainly will invest heavily into one DE (upon successful campaign that decision will be made) but we will also work with all interested parties and devote some resources to get other up (it is the same in case someone is trying to bring up other OS or DE, we will try to help such effort as much as we can). The point is to give to community working Free phone and finally disrupt the status quo.
How about you give the community a working free phone by shipping a phone that runs vanilla AOSP and try evolving it into what you think you want it to be? At least that way, when Linux-desktop-on-the-phone proves itself (again) to be an even worse failure than Linux-on-the-desktop, the world will still be left with a free phone, instead of a free phone that strangled by its associated with a doomed software strategy.
On the phone we plan to produce, you will be able to install Android as matter of fact - it is just that we will not do it by our default as we want a pure Linux stack that is going to roll in updates normally as desktop is and not being attached to older kernels and heavy forks of ecosystem. Even for scale of Google it is not feasible to do it that way so they are now doing their own research and development (Fuchsia OS or something like that).