Here's what I got from the testing.
With the fresh air blocked..."
"The other intake filter is inside the side compartment next to the hydraulic tank, etc..." 160-170
I understand the dealer has the machine to work on, but I just wanted to check a few things.
Air filters into the evaporator/blower unit: from your post quoted, I would like to check which filter you blocked off?
The one that sits in the back of the box is the "outside air" filter that pulls air from the hydraulic tank area, this is the one to block off in summer. It's very hot air coming through there.
The filter that sits in the cover of the unit, recirculates air from the cab down by by your feet, this is the one to use in summer. Keep the filter clean as it's a dusty place next to your boots.
The other thing is the design and sealing of the unit. The "blower" fan, looks like it's a "sucker"? With the intake filters the other side of the heater / evaporator, the air gets pulled through the filters, through the cores, through the "fan" (sucker), then up through some vents? So the cores need to be sealed around to the plastic casing, with some foam insulation and tape/glue, so that the air coming through the filters can only go through the cores to get sucked through the fan. As the photos look, air will take the easiest route which is to go scooting around the whole core and barely through it at all.
If you got the picture already from the other guys help, then apologies for being repetitive, but I got the feeling that you might have blocked the wrong filter and not got the importance of the sealed wall the cores need to be in the middle of that box.
Cheers. Hope you get the thing working properly.