That's what I was thinking. Arch switched to all x86_64 a few months ago (announced the phase-out) but, Arch would probably be just as lightweight. I'm not sure what DE it comes with
None, the default install of Arch leaves you with a terminal prompt and basic utilities. You can install whatever you like from there. It's closer to something like the Debian minimal install than Ubuntu desktop.
It doesn't come with one either, it's for servers. You can install the ubuntu-desktop package (which is basically what you get off of the desktop install disk) afterwards, though. Not really clear if that will still be option on i386, that is do a net install and then install ubuntu-desktop. It sounds to me like they are just retiring the installer images for i386 and not the desktop related packages, so if you really need it then you can go this route. I bet it won't be well tested/supported though.
On a related note, Unity is getting the axe and 17.10 will ship GNOME as the default DE.
