1. It is a very bad idea to cut though the double top plates! It causes the stability of your house’s structure ! It won’t cause an immediate collapse but it is still risky when there is a strong wind or a earthquake! If you really need to do it then you need two pieces of steel bars that are jointing the broken plates for reinforcement! If your house has many registers on the wall then you may have this stability structure issue! 

2. To install a duct inside a wall is a bad idea! The ductwork should go from either the attic or the basement! If you do not have a basement, the you need to add additional structures on the edge of ceiling on first floor for your ducts! - actually this is the best way for a ductwork - it saves energy!

3. It is a bad idea to use the wall as the other half of your ductwork. It causes an energy leak!  A ductwork must be completely sealed and it should be constructed with two pieces of sheet metals!

4. Never using the room between two poles of the wall as a returning air duct, it may cause a lung cancer because ducts will suck insulation fiberglass dusts into your house every day! Think about it, the air filter can not block these tiny dusts so you are breathing these harmful dusts every time when the fan is turned on.