Seeking guidance getting simple Volto-frontend install working behind nginx TLS/SSL

I think that what you are missing is that if you have SSL, you have to amend the rewrite accordingly like:

rewrite ^/(\+\+api\+\+\/?)+($|/.*) /VirtualHostBase/https/$server_name:443/Plone/++api++/VirtualHostRoot/$2 break;

More on Zoe's VHM rewrites: 20. Virtual Hosting Services — Zope documentation 5.7 documentation

I recommend that you use "Seamless mode":

Also, take a look at the cookiecutter template, where there is a lot of knowledge poured on it:

I hope this helps!