Thank you for your response.
After all that I was able to connect to the Internet through my iPhone through tethering, after installing a few files. Then I was able to get Yaourt installed, and did install the driver from AUR: broadcom-wl.
I’m not sure how it came about that when I subsequently ran the installer, it stalled out on the download of the first package. When I cancelled the install, and tried to run the installer again, I got the message that there had been an installation error and I would need to reboot.
I had specified the efi partition to be mounted in the partition selection step. I’m not sure if this was a mistake. As it turned out, though, when I reboot, the iMac will not put me back into refind or refit, the boot selection step. If I bail out of grub2, which is set to boot into Ubuntu by default, and try to select another OS to boot (ie, the USB drive), the system now boots into OS X. This, of course, is another issue.
Anyway, the question comes up of the difference between and Antergos install and an Arch install. With Arch, if I reboot I can start at the point where the previous session left off, providing I mount the various pieces and run chroot. I’m uncertain if there is any way to do this with the Antergos install. I want an Arch system, it’s the best, most bulletproof of any system I have installed in well over 20 years. Antergos is, I admit, seen as an easy way—a time saver—to get a running system; I have work to do!
There are a number of files now visible in the partition I had specified for install, like the pacman.d that had been used, /var, and a stub for /boot/efi.
So, after all this, my question has to do with the way the installation interacts with anything I do on my own to get a driver installed for this broadcom wireless adaptor, just to be able to complete the install. Probably I need to ask: What is the best sequence of steps to start the antergos install? At what point to do these tweaks needed to install the driver? I have seen several suggestions as to how to get a driver installed on an Arch system. One, as was suggested here, is to load up the required files on a USB device. To some extent this is what I did. I was encouraged to see a network connection working out of the box, once the tether had been established by a few arcane magical steps that I will not repeat, save that an ifuse mount had to be made.
I went through a bunch of steps (copying over *db files to /var/lib/pacman/sync (which are now permanently on the partition and were—even though up to date—updated several times over a slow tether connection to enable me to install broadcom-wl via yaourt (not to mention the installation of yaourt via a repo). Once these steps had been made, I had used the networkmanager applet to change to the broadcom wireless connection. No problem.
But how did the installer handle this? What did I do that resulted in the installer not being able to actually download the packages and install them?
Perhaps I’ll try the Arch installation steps if necessary, but I hope to use Antergos.