I love you, guys!!!
I fixed the problem with microG’s signature spoofing by making a symbolic link as @bundyo suggested:
devel-su
[Type your password]
ln -s /home/defaultuser /home/nemo
# As he has pointed out, move/backup files if you already got a /home/nemo folder
And then, fixed the WhatsApp issue by changing ~/android_storage/WhatsApp owner, as @Alex said:
devel-su
[Type your password]
chown -R media_rw:media_rw /home/defaultuser/android_storage/WhatsApp
Sailfish community never disappoints me. Now, let’s donate some chips to microG developers.