Looking at the log and what files are transferred with adb, these are the commands that should be possible to execute manually.
Boot to bootloader
fastboot flash recovery ~/.cache/ubports/images/FP2/recovery-FP2.img
fastboot flash splash ~/.cache/ubports/images/FP2/splash-FP2.img
Boot to recovery
adb push ~/.cache/ubports/pool/ubports-b68401f63e2fac3c8c533a804183ee276ee834b0154ca3ec20c80504dd17415c.tar.xz /cache/recovery/
adb push ~/.cache/ubports/pool/ubports-b68401f63e2fac3c8c533a804183ee276ee834b0154ca3ec20c80504dd17415c.tar.xz.asc /cache/recovery/
adb push ~/.cache/ubports/pool/device-10ac49deff49c650c78d8c8139c6099a7e1058649a318dfc6f668a08fba9382a.tar.xz /cache/recovery/
adb push ~/.cache/ubports/pool/device-10ac49deff49c650c78d8c8139c6099a7e1058649a318dfc6f668a08fba9382a.tar.xz.asc /cache/recovery/
adb push ~/.cache/ubports/pool/keyring-4c4e7ef380ebcfa2c31084efa199138e93bfed8fc58aa3eb06bdf75a78af9b57.tar.xz /cache/recovery/
adb push ~/.cache/ubports/pool/keyring-4c4e7ef380ebcfa2c31084efa199138e93bfed8fc58aa3eb06bdf75a78af9b57.tar.xz.asc /cache/recovery/
adb push ~/.cache/ubports/pool/version-697.tar.xz /cache/recovery/
adb push ~/.cache/ubports/pool/version-697.tar.xz.asc /cache/recovery/
adb push ~/.cache/ubports/gpg/image-signing.tar.xz /cache/recovery/
adb push ~/.cache/ubports/gpg/image-signing.tar.xz.asc /cache/recovery/
adb push ~/.cache/ubports/gpg/image-master.tar.xz /cache/recovery/
adb push ~/.cache/ubports/gpg/image-master.tar.xz.asc /cache/recovery/
adb push ~/.cache/ubports/commandfile/ubuntu_commandFP29161 /cache/recovery/ubuntu_command
Got this by parsing the log I posted above via
grep "8 push" ~/.cache/ubports/ubports-installer.log | sed -e "s/.*exec cmd //; s/ -P 5038//; s/\\\\\"//g; s/\",\".*//"
But I’m not fully sure where to get these files from when not using the installer at all.
The fastboot images are specified at https://devices.ubports.com/api/installer/FP2.
Some of the actual install files come from http://system-image.ubports.com/ubports-touch/16.04/devel/FP2/index.json
Make sure to look at the entry with type: "full", otherwise the files starting with “ubports” are missing.
The gpg files come from http://system-image.ubports.com/gpg/
And the commands file is generated by the installer. For my last run it looks like this
format system
load_keyring image-master.tar.xz image-master.tar.xz.asc
load_keyring image-signing.tar.xz image-signing.tar.xz.asc
mount system
update ubports-b68401f63e2fac3c8c533a804183ee276ee834b0154ca3ec20c80504dd17415c.tar.xz ubports-b68401f63e2fac3c8c533a804183ee276ee834b0154ca3ec20c80504dd17415c.tar.xz.asc
update device-10ac49deff49c650c78d8c8139c6099a7e1058649a318dfc6f668a08fba9382a.tar.xz device-10ac49deff49c650c78d8c8139c6099a7e1058649a318dfc6f668a08fba9382a.tar.xz.asc
update keyring-4c4e7ef380ebcfa2c31084efa199138e93bfed8fc58aa3eb06bdf75a78af9b57.tar.xz keyring-4c4e7ef380ebcfa2c31084efa199138e93bfed8fc58aa3eb06bdf75a78af9b57.tar.xz.asc
update version-697.tar.xz version-697.tar.xz.asc
unmount system
If you want to wipe the user data as well, add the line format data after format system.
So basically it should be possible to do all this without the installer now 