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It really strikes me that you've got a heater control valve that isn't able to close all the way. I can't at all see why flaps in the air distribution box are involved to the extent you indicate. When the valve is shut, coolant should completely stop flowing through the heater core and vented air should quickly return to near-ambient conditions no matter which flaps are open or shut. The amount of residual heat that can backflow by convection into the heater core should be minimal.
First off, are you sure you installed the valve in the correct flow direction? There is an arrow on the body, it should point toward the heater core. Although it shouldn't make a huge difference, the pressure under flow in the wrong direction may be opening the valve plate a little and letting hot coolant trickle by. There could also be foreign matter that got flushed forward and stuck in the valve. When closed and the heater core allowed to cool a bit, both the inlet and outlet nipple areas should be noticeably less hot than before. I would definitely consider removing the heater control valve for closer inspection and verification of operation and closure. Pinch off the hoses to minimize fluid loss. To avoid damage, pinch over an area wrapped in a rag -mind you if a hose does split then that's probably a good thing as you just prevented accidental failure down the road.
On additional thoughts, make sure you're getting a proper vacuum seal at the heater control valve vacuum nipple and the vacuum tubing isn't damaged. Check the rubber connector sleeve to see if it has a tiny split anywhere along its length. Check the exposed length of the vacuuum tubing to make sure it isn't damaged, like being allowed to ride on the block or touch a hot manifold -use some old heater hose or spiral wire wrap as a protective shield if needed. Plastic standoff clips are used to keep the valve steady and the coolant hoses away from the block, each other and the dipsticks -these standoffs often break, you can use multiple black (UV proof) nylon zip ties to fashion your own standoff, although they may get brittle and break after a few years.
Most importantly, most of the newer design heater valves (including Volvo OEM) have a vacuum nipple with a smaller O.D.and are set at a steep downward angle compared to the original. There is a small rubber adapter elbow available as both a Volvo and aftermarket part. If you use it, it's often way too loose a fit. I simply reuse the old rubber connector sleeve by pulling it off and turning it around (best to seal it with RTV or something similar so it doesn't slip off as long as you absolutely don't get any inside the vacuum tube or nipple). The bend at the bottom will now be quite sharp, so to avoid kinking the vacuum tubing I slip over a short length of small O.D. vinyl tubing.
BTW, with a turbo, if the interior firewall is unusually warm at the passenger footwell then make sure the aluminum heat shield behind the exhaust manifold area on the firewall is in place and there is a suitable air gap behind most of it. I've seen engines where the heat shield has been torn off for whatever reason. Non-turbos do not have this shield.
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Dave -still with 940's, prev 740/240/140/120 You'd think I'd have learned by now
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