I just fixed my Spotlight index issue a couple days ago with the following commands. Was driving me nuts too.
cd /
sudo mdutil -E /
sudo mdutil -a -i off
sudo rm -fr .Spotlight-V100/
sudo mdutil -i on /Volumes/Macintosh\ HD
For MacOS 10.15 Catalina and beyond, the last two lines should be:
sudo rm -fr /System/Volumes/Data/.Spotlight-V100/
sudo mdutil -i on /System/Volumes/Data
I know you tried something similar with Monomeeth guidance but maybe the subtle difference will work for you.
I added the actual Volume directory and Name to the last command since I had some USB drives plugged in that would get indexed otherwise.
I used Alfred for a while too but was happier with Quicksilver so I switched back.
Noticed you are running macOS 10.13.6 beta. Any chance that could be causing an issue? I would have put this as a comment but I don't have the rep yet. :-(
