If you can, I would make 2 gallons of blueberry wine, because once the wine has time to settle out, you are going to lose some wine due to sediment and by making 2 gallons you will have extra wine to top up your 1 gallon jug.
Potassium Sorbate will not stop an active fermentation, it only prevents a wine from re-fermentation. You add Potassium Sorbate to finished wines before bottling to reduce the possibility of re-fermentation.
How To Stop A Fermentation
If the wine is really as bad as you say... drain cleaner! I made 5 gallons of watermelon wine that turned out horrible, tasted like an old rind even after it was aged for a year, so I politely made my way to the sink and down it went.
As long as the bucket is made of food grade plastic and you can install a bung and airlock on the lid to keep air out. I don't really care to use plastic as a secondary, but that's just me.