You know, for what it's worth, I had a leaking heater core about 5 years ago. It was leaking into the driver side foot well and became an air freshener I, nor my passengers, didn't care for. Mechanic said it needed to be replaced and recommended against those quick fix solutions. Not having the budget nor the time, I went ahead anyway and poured in a radiator leak fix solution (the one with the glittery bits) with the intent to replace it at a later date. To my satisfaction, the leak stopped, the hot air came out of the vents hotter and I haven't had the need to replace it (again, that was 5 years ago).
Not sure if I was just lucky or if the particulars are different, but I figured I should share my experience.
Best of luck with your Montero Challenge of the Moment!