yes.
to all of your questions.
i suggests to disconnect the rubber fitting between the evaporator box and drain line, and just give it a nice shot of compressed air.
not sure about the spray foams.
all ducts can be removed relatively easy and clean if needed.
you can also acess the inside of the air box from 3 different locations.
1. cabin air filler housing.
2. removing the heater core (will give you access to other side of box)
3. removing the vent fan.
you can check out the attached links to better understand the internals of the evaporator box, and the AC ducts system.
Step 1 -Heater removal:
https://www.dropbox.com/s/hxb4w0y3incjrcc/step%201%20heater%20core%20removal.pdf
Step 2 - Dashboard cover removal
https://www.dropbox.com/s/bjsmzjkt7fz4jru/step%202%20Dashboard%20cover%20removal.pdf
Step 3 - Evaporator replacement:
https://www.dropbox.com/s/wjz9stc83j3wxpe/step%203%20Evaporator%20replacement.pdf
you obviously don't have to go thru all that just to give it a good clean, but it will give you a better picture.