The thing is, though, that you talk about "messing around with DHCP or DNS" when DNS is the exact tool you need to do what you need to do: have an external URL resolve to an internal IP address for devices on the LAN.
Talking about doing stuff with htaccess, redirects and virtualhosts sounds like the kind of fix that gets put in because it seemed sensible at the time, but ends up being a millstone round your or somebody elses neck for the next few years. It'll just be that thing that somebody put in in the past, but nobody ever has the time or inclination to take it apart and redo it properly.
Setting up an internal DNS server wouldn't have any effect on the accessibility of the web server from the internet (because your port-forwarding is already being done by IP address), but it would give you the simplest, most understandable way of accessing the server inside the LAN.
Basically, and I'm not trying to be nasty, because I know the kind of pressure that gets put on technical people in small companies, but you're bodging a fix rather than spending a little more time on it and getting a much better and more capable solution.