@mason, I second your query to @ekohl. The confusion is not only around syntax errors. For eg:
8 hours ago, I started “anew” on the installer.
“foreman-installer -i --scenario katello -–foreman-proxy-dhcp=true --foreman-proxy-dns=true” completed successfully and reflected the DHCP & DNS features as being active on the proxy (visible via web interface → Infrastructure > Smart Proxies → localhost) !
Naturally I did do a quick dance around my chair. 
But this was only the start. 
I then started applying config changes one-by-one, adding only a single option after every successful foreman-installer run, validating the config change with “foreman-installer --help | less”
IPv6 addresses (with or without […]) is definitely not accepted by the foreman-installer which I believe only caters for IPv4 in the DHCP proxy. With FQDN’s being accepted, I’m hoping for a manual DHCP6 configuration workaround a bit later…
… with the DNS proxy though, the foreman-installer happily accepts IPv6 addresses i.e.
–forman-proxy-dns-forwarders=2001:4860:4860::8888
though being reflected as [“2001:4860:4860::8888”] when being validated. Yet now I ponder the reasoning for the quotes (") within the brackets ([…]) than being on the outside as one would’ve expected with IPv6 ? 
Marching on, the “installer initialization string” at this point thus being :
**foreman-installer -i --scenario katello **
**–foreman-proxy-dhcp=true **
**–foreman-proxy-dhcp-interface=ens160 **
**–foreman-proxy-dhcp-gateway=gateway.domain.com **
**–foreman-proxy-dhcp-nameservers=foreman.domain.com **
**–foreman-proxy-dns=true **
**–foreman-proxy-dns-interface=ens160 **
**–foreman-proxy-dns-zone=domain.com **
**–foreman-proxy-dns-forwarders=2001:4860:4860::8888 **
–foreman-proxy-foreman-base-url=https://foreman.domain.com
Yet … the moment I started to add TFTP options, the DHCP proxy started to fail with
“Couldn’t enable ‘dhcp_isc’: Invalid IP Address gateway.domain.com”
and within
/var/log/foreman-proxy/proxy.log
" Disabling all modules in the group [‘dhcp_isc’, ‘dhcp’] due to a failure in one of them: Invalid IP Address gateway.domain.com"
The host is still dual stacked (IPv4 & IPv6) and gateway.domain.com resolves correctly to the hosts’s IPv4 address… changing the FQDN to the IPv4 address, doesn’t resolved the issue …
and /etc/hosts has all references to 127.0.0.1 and ::1 removed.
** For clarity, once “foreman-installer -i --scenario katello -–foreman-proxy-dhcp=true --foreman-proxy-dns=true” completed successfully, a single option was tacked on per installer run, until the noted error was experienced.
The DHCP config being:
--foreman-proxy-dhcp Enable DHCP feature (current: true) --foreman-proxy-dhcp-additional-interfaces Additional DHCP listen interfaces (in addition to dhcp_interface). Note: as opposed to dhcp_interface additional subnets using `dhcp::pool` and related resource types (provided by the theforeman/puppet-dhcp --foreman-proxy-dhcp-config DHCP config file path (current: "/etc/dhcp/dhcpd.conf") --foreman-proxy-dhcp-gateway DHCP pool gateway (current: "10.10.0.1") --foreman-proxy-dhcp-interface DHCP listen interface (current: "ens160") --foreman-proxy-dhcp-key-name DHCP key name (current: UNDEF) --foreman-proxy-dhcp-key-secret DHCP password (current: UNDEF) --foreman-proxy-dhcp-leases DHCP leases file (current: "/var/lib/dhcpd/dhcpd.leases") --foreman-proxy-dhcp-listen-on DHCP proxy to listen on https, http, or both (current: "https") --foreman-proxy-dhcp-managed DHCP is managed by Foreman proxy (current: true) --foreman-proxy-dhcp-nameservers DHCP nameservers, comma-separated (current: "10.10.0.121") --foreman-proxy-dhcp-netmask DHCP server netmask value, defaults otherwise to value based on IP of dhcp_interface (current: UNDEF) --foreman-proxy-dhcp-network DHCP server network value, defaults otherwise to value based on IP of dhcp_interface (current: UNDEF) --foreman-proxy-dhcp-node-type DHCP node type (current: "standalone") --foreman-proxy-dhcp-omapi-port DHCP server OMAPI port (current: 7911) --foreman-proxy-dhcp-option-domain DHCP use the dhcpd config option domain-name (current: ["pxecloud.com"]) --foreman-proxy-dhcp-peer-address The other DHCP servers address (current: UNDEF) --foreman-proxy-dhcp-provider DHCP provider (current: "isc") --foreman-proxy-dhcp-pxefilename DHCP "filename" value, defaults otherwise to pxelinux.0 (current: "pxelinux.0") --foreman-proxy-dhcp-pxeserver DHCP "next-server" value, defaults otherwise to IP of dhcp_interface (current: UNDEF) --foreman-proxy-dhcp-range Space-separated DHCP pool range (current: UNDEF) --foreman-proxy-dhcp-search-domains DHCP search domains option (current: UNDEF) --foreman-proxy-dhcp-server Address of DHCP server to manage (current: "127.0.0.1") --foreman-proxy-dhcp-subnets Subnets list to restrict DHCP management to (current: [])
Then I tacked on “–foreman-proxy-dhcp-server=10.10.0.121” and life is all funky again 
The “installer initialization string” now mutating to:
**foreman-installer -i --scenario katello **
**–enable-foreman-proxy **
**–foreman-proxy-dhcp=true **
**–foreman-proxy-dhcp-server=10.10.0.121 **
**–foreman-proxy-dhcp-interface=ens160 **
**–foreman-proxy-dhcp-gateway=10.10.0.1 **
**–foreman-proxy-dhcp-nameservers=10.10.0.121 **
**–foreman-proxy-dns=true **
**–foreman-proxy-dns-interface=ens160 **
**–foreman-proxy-dns-zone=domain.com **
**–foreman-proxy-dns-forwarders=2001:4860:4860::8888 **
–foreman-proxy-foreman-base-url=https://foreman.domain.com
… and so the journey continues, still eluding the once off single installer line … and acquiring clarity.