I have one of those portable beer chargers and they WILL NOT carb beer. They produce enough pressure to push beer, but it would literally take about 15 of the small CO2 canisters to add any appreciable CO2 to a 5 gallon corny, let alone a 15.5 gallon keg.
You would need AT LEAST a 20oz CO2 canister to carb 5 gallons of beer, and that would cost about $25 for the canister, another $15-20 for the regulator, a couple bucks for tubing, about $8-10 for the CO2....all of which definitely equals a pretty poor ROI just to carbonate some old, oxidized beer.
I own all of this type of equipment, and I wouldn't even waste my CO2 trying to revive this beer.
Uncarbed beer isn't horrible if you get it cold enough. Lots of Europeans even prefer their beers served uncarbed or very lightly carbed, furkin/cask conditioned style.
I think in the end, you just need to let it go