Can't downgrade Linux kernel: not enough free disk space

I'm stuck in an archiso/chrooted environment (booting from a USB flash drive) because my Wi-Fi drivers are incompatible with Linux 4.8.7-1, and cause the system to hang at boot. I'm trying a few things right now: - First of all, I got rid of the b43-firmware via yaourt -R after chrooting into my mounted internal hard drive. The uninstall was successful as far as I know; no errors or warnings were raised. I can still access the internet via an Ethernet dongle at this point. - Unfortunately, this caused an error where, during boot, after the grub menu, I got the following error messages: b43-phy0 ERROR: Firmware file "b43/ucode29_mimo.fw" not found b43-phy0 ERROR: Firmware file "b43-open/ucode29_mimo.fw" not found b43-phy0 ERROR: You must go to http://wireless.kernel.org/users/Drivers/b43#devicefirmware and download the correct firmware... - I'm now trying to downgrade my kernel back to 4.8.6-1, but I'm having some trouble with that too. After chrooting into my mounted root partition (which is about 200GB; about 10GB used), I cd into /var/cache/pacman/pkg and try to pacman -U linux-4.8.6-1-x86_64.pkg.tar.xz linux-headers-4.8.6-1.pkg.tar.xz, but for some reason I get these errors: error: could not open file: /etc/mtab: No such file or directory error: could not determine filesystem mount points error: not enough free disk space Now, the first error is just plain wrong. /etc/mtab is available as a symlink to /proc/self/mounts; I'm not sure what the second error is even talking about; and the third error is also straight up wrong: a Linux kernel downgrade shouldn't use over 190GB of space. At this point, I've also tried to reinstall b43-firmware via yaourt in the chrooted environment, but no matter what package I try to install, I get something along the lines of target not found, even when the package shows up via yaourt -Ss. It's also worth noting that Yaourt's interactive environment doesn't actually work inside chroot. How can I either: - Resolve the issues preventing me from downgrading the kernel; or - Resolve the issues with the wireless drivers which are keeping me from booting?