The isolated install uses the prebuilt binaries. Kovid only pre-builds binaries for i686/x86_64.
Building calibre from source works just fine!
The password-protected login issues are a result of
ArchLinux bug #43382 -- the solution is to rebuild the package yourself, and edit the PKGBUILD to no longer strip out the heavily-modified libs that are bundled with calibre.
Thank you ArchLinux developers for your care and concern!

They don't want to try getting those modifications pushed upstream. They don't want to stop patching out calibre. They don't want to drop calibre to the AUR, so people will be forewarned.
Idealistic purity wins out over user experience...