Looking at this setup, I understand why you want to use FreedomBox with HTTP only. My solution in the other thread should do that job.
So, I don’t see major issues with why this setup would not work. I tried this and it work as follows:
On Host machine:
$ apt install apache2
$ a2enmod http
$ a2enmod http_proxy
$ a2enmod headers
$ cat /etc/apache2/conf-available/proxy.conf
ProxyPass / http://127.0.0.1:8080/
ProxyPassReverse / http://127.0.0.1:8080/
RequestHeader set X-Forwarded-Proto "https"
ProxyPreserveHost on
$ a2enconf proxy
$ systemctl restart apache2
On FreedomBox:
$ a2dissite plinth-ssl
$ systemctl apache2 restart
$ vi /etc/plinth/plinth.conf (Set use_x_forwarded_host = False)
$ systemctl restart plinth
On browser:
http://localhost/plinth/
(worked)
I haven’t evaluated the security implications of setting use_x_forwarded_host = False. So, do your own research on that.