Before the fix attempt I measured almost 150 F at the leftmost vent and only 70F at the rightmost with auto climate set to 89, outside air, and upper vents only.
If you are not aware, there is a definite issue with the factory heater cores in 2014-2015 Mirages becoming internally restricted. This restriction primarily affects the passenger side of the heater core. The main symptom is that the driver's side vents blow hot/warm air while the passenger side vents blow warm/cool air (the EXACT symptoms you are seeing). You can read more about the issue here.
That's great that you got positive results by back-flushing. It sounds like your cooling system was somewhat neglected...or possibly that the wrong coolant was used to top off your system at some point in the past. I don't think anyone has flushed their cooling system and described the old coolant as "brown crap" lol.
If you install a brand new heater core in a Mirage, you will see vent temperatures approaching 150 degrees across ALL of the dash vents. So while your heat has gotten better, there's probably still some of that mystery goo that has collected on the passenger side of the core. Nobody has come up with a magic fix for that yet.
My own car is like this. You can read my story here. My heat is not perfect but its fine 99% of the year. Do I pull my dash apart to fix a problem like that? I'm not ready to go there just yet. But I don't live where it gets really cold in the winter, either.
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View my fuel log 2015 Mirage ES 1.2 manual: 52.2 mpg (US) ... 22.2 km/L ... 4.5 L/100 km ... 62.6 mpg (Imp)