Pre-hung French Door install

Hello everyone. I have a pre-hung french door that I am trying to install. I need about 1 inch of width more than what the opening is currently. The height will work ok. looking up into the doorway, I can see the header, with a 1/2" space, and the cripples above that. There is the standard jack and king studs on either side. If you look, there is a HVAC vent that has been cut into the header. This house was built in the 70's. I find it odd that this might be a load bearing wall with a cutout for the vent. on the other side of the vent is the ducting. i would assume both sides of the header have been cutout. How can i go about getting an inch? shave 1/2 inch off both jack studs? This is on the first floor of a two story.

thanks for any help you can give.

EDIT: I forgot to mention, the last picture shows a hole i cut and can see a stud just a few inches away from the king stud. not sure if that matters.

 

If it's a load bearing wall you need a longer header, and that vent cannot be there.

 

How can I confirm if it's load bearing? From the framing, it seems so, but from the AC ducting and vent not so much....