If I’m reading #4 correctly, stuffing any kind of blanket or such into the inside of an air vent as a longterm solution would be a big fat no-no. There will be far too much moisture buildup inside the vent system during normal use and it won’t be long before you can have a serious mold problem on your hands that will require professional remediation that would likely not be cheap.
The best ducting from a noise transmission pov, that is, noise transmission along the length of the duct itself, is the foil kind. They are cheap and quiet, a bit delicate maybe, but if anything happens to it, it can be replaced easily enough...much quieter than the sheet metal, boxed ducting which would require all kinds of noise insulation on the outside of the ducts and they still might not be as quiet as the foil type. But, that only concerns noise transmission along the duct material itself, not along the airway inside it. For that, I’m not really aware of a workable solution unless you’re able to reroute your ducting to a degree and can exit your room air to another area of the house least bothered by the noise, either straight to the HVAC return, or, if there is sufficient return airflow from it, to another room.
The best ducting from a noise transmission pov, that is, noise transmission along the length of the duct itself, is the foil kind. They are cheap and quiet, a bit delicate maybe, but if anything happens to it, it can be replaced easily enough...much quieter than the sheet metal, boxed ducting which would require all kinds of noise insulation on the outside of the ducts and they still might not be as quiet as the foil type. But, that only concerns noise transmission along the duct material itself, not along the airway inside it. For that, I’m not really aware of a workable solution unless you’re able to reroute your ducting to a degree and can exit your room air to another area of the house least bothered by the noise, either straight to the HVAC return, or, if there is sufficient return airflow from it, to another room.