Thanks for doing further testing on this.
I had to install a different patched version of the Meslo font for powerlevel10k (a ZSH theme), and Core Shell saw that without issue after being installed via Font Book. I did have to quit and restart Core Shell to get it to pick up the Powerlevel font.
I may have just bungled installing the Powerline fonts via Font Book, or something else could have glitched. Everything worked as you said it should with the Powerlevel font.
I'm really glad to hear the Powerline fonts are getting embedded into the next version of Core Shell. My favorite Terminal emulator keeps getting better. 