If I slooshed my mouth with gin, so that my breath totally ponked of alcohol, but sat it all out and didnt swallow any, would a breathalyser think I'd been drinking?
Yes it would think you had been drinking. If the police breathalise you they ask you when you had your last drink. You should not be breathalised till 20 minutes has elapsed from then.
I have a pretty decent breathalyser, digital but not police grade. If I use Listerine mouthwash and breathalyse myself a few minutes later it registers not far off double the limit.