sorry for overlooking here, its absurd that they say its a forever problem, it the company designed it with ac then it would run with ac and not overheat, mine is 1996 i use ac all the time even while idling for half and hour and temp gauge goes to half in stop start traffic and slightly below half while cruising at speed,
now your problem, first of all get the thermostat valve back or your engine will die soon, mini pajero are especially nazuk when it comes to operating temp, check the valve before putting in, from the pics yes the radiator is original mitsubishi part however change the cap i tried this cap in mine and the valves wouldn't work properly, suggest you to try different 0.9 bar caps, also the size seems bigger then what is required i think the problem would be solved by changing the cap as you have already changed the radiator,