yup, you'll have eliminated the heater core entirely, so you can narrow it down to whether its that or something else. Even if it is something else as well, if its leaking through the cabin, then you'll need to replace it at some point if you want your heater back.
yeah.. if you get access down there, or rig up a hose and a couple of hose<->hose connectors just where it goes through the firewall. Probably easier 2nd way.
on an rb25 the thermostat is a bit easier to change than rb20.. but its located just where the bottom radiator hose goes into the engine.