Is the Librem 5 planned to support Arch Linux with KDE? Or Manjaro KDE on it? If not, how difficult is it to port Manjaro-KDE/Arch-KDE to it?
In principle, yes. Purism is upstreaming all their Librem 5 work, so other distros will benefit. Purism is also partnered with GNOME, KDE, and UBPorts with the goal of having GNOME, Plasma Mobile, and Ubuntu Touch work well on the Librem 5, so the short answer is that Arch/Manjaro with KDE (or at least Plasma Mobile) will work.
A brief post is here:
But Purism won’t spend too much of their own time supporting those distros, and their focus is on GNOME, so more of the load will be on those respective communities to make things work.
The Librem 5 more than likely won’t work with standard PC OS installation images, since those rely on a different firmware standard and a different processor architecture, which (at least partially) defines what constitutes a bootable image or bootable storage media.
However, each distro should be able to choose to produce and maintain an installer/image/package that will work on the Librem 5. I’m optimistic that at least a handful of distros will see fit to put in the effort to do this, as Purism will have done most of the work for them and hopefully it is a relatively popular product that’s worth supporting. Obviously the user base for such an installer will be small compared to x86_64, but I would imagine that the Librem 5 will be a doddle to support and more popular, compared to some of the obscure hardware that Debian has installers for.
Then, having supported the hardware, each distro will also need to package phone-appropriate software, such as diallers and messaging apps and a phone-appropriate GUI shell. The easiest route would be to package the software that’s being developed for PureOS, or KDE’s Plasma Mobile. Or they could use something else, or a mixture.