Can I install apps to adopted SD card from recovery? WhatsApp is stubborn about not installing to SD card. I have an unlocked bootloader, but have not rooted or installed a custom recovery yet. I am trying to avoid rooting because of SafetyNet, and because I need it pretty much only for force-moving WhatsApp (etc.) to an adopted SD card.

Since my bootloader is unlocked, is it not possible to:


boot into custom recovery without flashing it
create a flashable .zip containing WhatsApp 
instruct the installer to locate it in adopted storage, ignoring the app's installLocation setting


?

Why is it said that we need root for force moving apps, when the bootloader being unlocked should be enough? I don't have a problem with flashing a custom recovery, just ... why flash when one-time boot should be sufficient?

Likely I'm missing some limitations of fastboot and the recovery here.

Edit: Trying out @DanHalli's answer gave me an error

P:\Software\Management\SmartPhone\apks&gt;adb install -s WhatsApp.apk
3644 KB/s (38480037 bytes in 10.311s)
        pkg: /sdcard/tmp/WhatsApp.apk
Failure [INSTALL_FAILED_INSUFFICIENT_STORAGE]
If you have the APK for WhatsApp on a computer and USB debugging on you can do 

adb install -s path/to/APK

I might have my syntax wrong, if you have it already installed you might need to add a -r after the -s  to have it reinstall, I've noticed it works kind of like a force install