[Sony Xperia XZ2 Compact Questions & Answers] orapyt:
Just to give closure to this: I did get eXistenZ on my Sony XZ2C Dual on build 52.0.A.8.107, and here is what I did exactly (could act as a guide, but please read completely before doing anything and I'm not responsible for any losses before, during, or after the process).

Prerequisites:

adb/fastboot (through package manager for Linux ('android-tools') or here for Windows)

mono (required only for Linux, through package manager)

Drivers (only required for Windows--install them from device manager when you have device turned on normally, and then again after you reboot into bootloader, as Windows detects it as a different device)

XperiaFirm

Newflasher

Steps:

I. Flashing the latest firmware:

Use XperiaFirm to get latest stock firmware with your customization (run with mono on Linux)

Delete userdata{somethingsomething}.sin from the firmware folder created by XperiaFirm

Place Newflasher into the folder

Shut-off phone

Hold volume-down button and plug in phone to computer to enter recovery mode

Run Newflasher and enter 'n' when prompted

Just wait until everything finishes (no data should be deleted).

II. Unlocking the bootloader:

Before you do, understand that you are voiding your warranty. Also it has some side-effects:

Get the unlock code for your device here.

Enable USB debugging from developer menu

Plug in phone to computer

Connect to fastboot:

adb reboot bootloader

Enter unlock code:

fastboot oem unlock 0x<insert your unlock code>

Wait for device to boot again (For me data was deleted. Also, don't mind the "can't check for corruption"; it's a side-effect of unlocking the bootloader)

III. Installing TWRP:

Get TWRP for XZ2

From the "Download," navigate to "Xperia XZ2 Compact/android_9/twrp/stock/" and download latest one

Extract, and open terminal in the extracted folder as a working directory

Plug in phone

Copy twrp{sometingsomething}.img to phone somewhere

Boot into TWRP:

adb reboot bootloader
fastboot --disable-verity --disable-verification flash vbmeta vbmeta.img
fastboot boot twrp{somethingsomething}.img

Enter pin if prompted

Go to Advanced, Install recovery ramdisk, and pick the copied twrp image (this will install twrp to recovery mode, so you just shutdown phone, hold volume-down button and power, wait for vibration, and let go to enter TWRP)

IV. Flashing other stuff (Magisk, permissive):

Copy Magisk .apk and .zip, and permissive .zip from the twrp .tar.gz downloaded above to your phone somewhere

Enter installed TWRP by going into recovery mode (shutdown phone, hold volume-down button and power, wait for vibration, and let go)

Go to install, pick the Magisk .zip, and slide to install

Reboot into system and install the Magisk .apk to make sure it's working (do not update Magisk as it may be unsupported; only use the version provided in the twrp .tar.gz)

Boot back into TWRP and install permissive the same way Magisk .zip was installed

Reboot into system and launch Magisk manager, go to modules to check if permissive is working

V. Installing eXistenZ:

Download the version for your firmware version

Place the .zip somewhere on your phone

Boot into TWRP (shutdown phone, hold volume-down button and power, wait for vibration, and let go)

Got to wipe, advanced, and selected only Data and Dalvik/ART Cache

Now install eXistenZ .zip like installing the Magisk .zip

Reboot into system

??? (takes a long time)

Profit!! (for me data was deleted, so I had to reinstall Magisk and permissive)

Notes:

You probably want to backup your data.

Personally, I used UK customization firmware because that is what my phone shipped with.

Some guides say remove the *.ta files from the firmware folder, but I did not and it worked fine??

For some reason, all fastboot commands return "FAILED (command write failed (Success))" on Linux (fastboot version 9.0.0_r41) for me, but worked on Windows??

If you get 'error getting bootctrl module,' in TWRP you might need to mount something... forgot what exactly (might be OEM)??

Edit 1: Added drivers for Windows