Sorry to hear about your problems. So just before you explore the world of corny kegs you might want to try this out. If your valve looks like this...
View attachment 87420
You might want to check the position of the black washer. Should be under the cap between the cap and the disc face of the valve...
View attachment 87421
With regards to pressure testing using beer and CO2 or water and CO2, CO2 will dissolve in both and reduce the pressure which will appear to be a leak but not. Best way to pressure test with water is with air, not easy on a King Keg unless you drill and fit a Schrader type valve (car/bicycle type valve).
I use sugar solution to carbonate my fermented beer in a King Keg. After fermentation in a bucket I cold crash for 3 days to clear the beer then transfer to my KK and add the priming solution. Leave for 2 weeks at 20C to carbonate and pressurise the KK. Another 2 weeks at 10C ish and it's ready to serve. You can do exactly the same in a corny keg rather than a KK. You will get a bit of sediment with both. With a corny the first glass will be cloudy but once the sediment is cleared from the dip tube you should be ok. I use a float tube in my KK and get clear beer right from the off. The sediment gets sucked up with the last glass.
Hope that helps