Curious to what "all" your refer to that talk about 2 lan interfaces with 1 wan interface.. its a router/firewall - your going to want 2 interfaces or how do you route between anything ;) Or firewall between anything? ;)
You could have 3 sure, you could have 6, etc. Where you have multiple WAN, and multiple LAN, etc.
If you put a gateway on the interface then pfsense would treat that as a WAN interface - so this would be connected to whatever network you have a gateway to get to other networks (internet normally) Your lan interface as pfsense would see it would be where your clients connect that want to get through to the networks on the other side of pfsense, ie pfsense's wan.
Sure you could use a captive portal to let them out or not, etc.