I also acidify my sparge water, and i have the idea my beer got better with it, we have very soft water here so i use about 10ml Lactic Acid on 6L of sparge water.
It will only slow it, the point of coldcrashing is just to clear the beer/cider after the fermentation is complete, if you wan't to completly stop it i think you need to pasturize it.
If you wan't a good sweeter beer you are to late and have to start over, change the maish schema so you got less fermentable sugars, and/or use a yeast that has a lower attenuation.
Adding sweetners or lactose won't result in anything nice in my opinion, better drink it dry!
It might actuality improve, they say climate change makes the alpha acids go down, that means brewers need to add more hops, and that means we actually will get more nice hop flavors!
Collecting CO2 in a balloon is good for coldcrashing, but i don't see how you can push out oxygen with it after dryhopping, what i do is just open it for a verry short time, and just also add a little sugar, that way new co2 will be generated to cover the beer.
I read that it is not the ph thats degrades, its the dodecylbenzenesulfonic acid, and that is there to break down the cell walls of the bacteria, it that is gone the acid won't be that effective.
For re-fermentation you only need like 1g/20L of beer, making bombs just depends on the sugar you add, the yeast quantity doesn't matter.
Maybe your supplier has Fermentis F2?, that would be better then nottigham.