OK, the heater control valve is job one. If it is also leaking, that could be a contributor to the smell but antifreeze smells sweet.

While the valve is being done, with parts of the dash torn apart, seriously consider doing the heater blower motor...or at least having the guy quote it. At the very least have them clean the debris (leaves, pine needles, etc) from the center console area where it accumulates coming in through the cowl vent. also, you can make sure that the cowl drains are working correctly and not clogged with the same debris (dump water into the cowl vent and look under either side of the car to verify that it is flowing out of the drains on either side right behind the wheel well).

As for the "motor" smell there are a lot of possibilities:

Air intrusion through a bad grommet in the firewall (clean the engine bay thoroughly and look at all the rubber grommets in the firewall).

Oil, dirt, old french fries, etc in the carpet and upholstery give an older Volvo a distinctive odor...it is a 23 year old car. The pad under the carpet might be saturated with water from wiper shaft leak or all the above smelly items.

Many other items I did not list...

The vents are manually closed when you want foot only, defrost only or defrost/foot only. There is not setting in the HVAC controls to accomplish this so the fact that the vents were closed is an indication of nothing in particular.

Just as a test, open all the vents, push the recirc button and make sure both the defrost and floor buttons are not depressed...now go for a drive. How is the smell? Now turn off the recirc and drive around. How is the smell and how is the ventilation even with the fan turned off?

While doing all this, with the motor idling play with all three buttons with the fan off. There should be short "pssst" as the vac motors move the flappers around but there should not be a continued sound of air leaking.

Mike