OK first thing to do is look at things the mechanical way.
#1: What is the max temp you have seen. If you do not know then go pick up a real gauge and drop it in so you have a number to go from.
You only need a cheep one unless you are going to keep it on there forever.

#2: Did you open the heater valve at fill up? if not it will always have trapped air tell you open it.

#3: Is the heater getting hot. If not something is plugged. I have seen the metal tube that runs around the right side of the engine get plugged or rust apart and leek. Stick a wire through everything or use light if you can. Heat is good to keep the window clear in winter. AC is a luxury item for the Snow Flakes. All D-50 and MM owners are sweat resistant and know the crank handles on the doors are the main climate controllers. LOL

#4: When checking the pressure you said it held 15 for 10 minutes after running. That is good. So no head problems there. If there was a problem it would have gone from 0 to 60 in nothing flat. And if it was a @ 15 and dropping after shutting down then it would have been going into the engine.

#5: The bubbles in the overflow is normal. It will always bubble tell every last drop of air is gone. It could take days for them to go away. OR if there is a hot spot on a cylinder wall it may always make some air. But that is not a problem to think about yet.

#6: Drop in a good Stant T-stat so you know it will work.

(TRICKS of the trade)
Drill a small pin hole in the flat flange of the new T-Stat. 1/16" or less is plenty, it's only air and will not affect how it works or cools. This will allow air to escape during filling. Some may already have the hole with a H bar in it. Coat the gasket lightly with some anti-seze before putting it together. It will make life easy the next time you take it apart 10 years from now. anti-seze is a wonder drug on gaskets, intakes, carbs, and water systems. Saves you time and makes it so you can reuse it if need be when on the road or at a camp spot hours from parts.

#6: Fan shroud. Does it have one? If not get one for a Montero early version. Any time a fan is more then 1 inch away it should have a shroud.

#7: Fan clutch. (1) Does it have any Resistance when you turn it? (2) Do you see any gray matter coming out of it? (3) Is the bi metal spring on the front of it missing? If no to #1 or yes to #2-3 replace it.

#8: last is the water pump. Does it have movement in the shaft or just the fan and clutch. The clutch will have movement. If you have access to a baroscope you can look in and see the impeller. I have seen impellers come loose from the shaft and I have also seen the area behind the impeller get worn and pitted from cavitation and bad water.