Basically the CO2 that was produced from the priming sugars has now been used up, so there is no longer any pressure in the keg, you managed to get half of the beer out before topping up with gas so that is quite good, it shows you don't have any leaks.
The little CO2 bulbs don't hold much gas at all so they don't last long, it works out quite an expensive way of dispensing beer. The Hambleton Bard S30 cylinders work out more cost effective.
An alternate solution is to take the lid off the keg and add some more sugar to reprime it, you will need to leave it in the warm for a week though for this to work. That should then let you dispense the rest of the beer.
Glugging air back through the tap is bad as that is allowing oxygen into your beer which will very quickly make it go off, so it's probably not worth trying to recover this keg, I'd just get it drunk quickly.