Might be a bit late to help here, but if you still need some assistance…?
The mouth is definitely used but there is no avoiding the fact you need to get those lung muscles working.
The diaphragm is where the power and tone come from imo. Practice using just your diaphragm to breathe for a while. Get it used to working independently. Lay on your back and put one hand over your belly and push it up and down with your breathing. Try not to let your chest move though.
With practice you will exercise and condition your internal breathing muscles more and your harp playing will improve.