Well, I finally got past the major hurdles so I’ll post my steps in case it helps someone else.
Even after flashing PQ1A.190105.090, I still had to specify the slot when flashing TWRP:
$ sudo fastboot flash boot_a twrp-3.2.3-0-mata.img
Touch wasn’t working in TWRP, but I didn’t want to flash a patched version, so I followed these directions to install TWRP on both slots. I had to push the TWRP installer to the device first, then install it with another adb command:
$ adb push twrp-installer-mata-3.2.3-0.zip /sdcard/
$ adb shell twrp install /sdcard/twrp-installer-mata-3.2.3-0.zip
After booting back into TWRP, touch still wasn’t working, but then I switched to the slot I was originally on and everything was fine. I was even able to make a backup 
Finally, in order to sideload the /e/ image, I had to restart the adb server with root permissions:
$ killall adb
$ sudo adb devices
Hopefully the instructions can be amended to include some of these differences.
PS: this is a minor gripe, but I noticed many (all?) of the instructions on the /e/ website have this as the last step:
- Once installation has finished, return to the main menu, tap Reboot, and then System
This should say “tap Reboot System, then swipe to confirm” or similar. There’s no need to return to the main menu as the reboot button is on the adb sideload screen.