I'm not sure I fully understand how to "publish slapos's apache" from existing backend to frontend (I've only ever programmed frontend-only apache websites),
You need a web server that works as a reverse proxy. I don't know if the erp5 installation script provides a frontend because slapos frontend system requires a public IP adress that is not usually free(need some cost to maintain it).
Anyway, it is easy to setup apache manually for backend erp5, use apache mod rewrite and mod proxy. See examples below. It is enough for a try.
https://lab.nexedi.com/yusei/a/blob/master/setup.sh
https://lab.nexedi.com/yusei/a/blob/master/erp5apachefront.conf
https://zope.readthedocs.io/en/latest/zopebook/VirtualHosting.html