'Is it possible that fermentation completed in the four days at 68F before I dropped the temp too low?'
It's definitely possible, though it's really impossible to tell without gravity readings. Bubbles in the airlock are a fairly unreliable way to gauge fermentation progress, as they can be cause by other things besides active fermentation (sometimes you even see no bubbles during fermentation, due to leaks).
'The brew has a very clean finish with good flavors.'
That's usually a good sign, but by no means a definitive indication of it being ready to bottle (a sign that fermentation has ended, that is).
'Should I bottle? '
Depends on if the beer's done fermenting. If not, there's no way to have good control over how much carbonation ends up in your beer. Get too much and you might have exploding bottles on your hands. Since you have a hydrometer now and you know it was 1.020 at the beginning of your experiment and you've pitched fresh yeast, if the gravity reading stays the same a few days straight, that tells you there are no more fermentable sugars and it's safe to bottle.
'Should I dump it?'
Do you want to drink it? You seem to indicate it tastes good. If so, don't dump it. Brewing beer is pretty safe, in that if you do it right you're all but guaranteed that what you have is safe to consume. Then it just comes down to whether you like the taste.
