I apologize in advance if this is an obvious fix that I’ve missed.
Is there a way that I can set up just one of my sub heading fonts (like H5) to be something other than the same font the rest are in? (like say a script like Cookie when other Headings are set to Oswale)? I don’t see that option in the Nirvana customizer which seems to override the WordPress.org theme customizer.
Nirvana does not use the customizer interface. You will find Nirvana’s settings under Appearance > Nirvana Settings (after installing the settings plugin).
The Heading Font option under Text Settings controls all the headings (H1-H6). To style a specific heading separately you’d need custom styling:
body #content .entry-content h5 {
font-family: YourFont;
}