I think the short version is that it's not easy, and if you want exact figures, tough, you're not likely to get them.
I think there's still a reasonable amount of sugar left when you drink it. If you doubt that, consider that when you're drinking it, it's all nice and pleasant, but if you leave it to ferment for another few weeks it keeps changing (sugar keeps being used up) and if you leave it a further month or two it will change more, so there is still a reasonable amount of sugar left even well after you could drink it. Whether that's 20% or 50% or even outside that range I don't know, but there's definitely more than 'almost none'. My guess is that more than half the sugar is gone by the time most people would want to drink it.
Testing using specific gravity isn't going to work well I wouldn't think. The specific gravity is going to be affected by alcohol and other things. With multiple things affecting the specific gravity you have no way to quantify how much any one specific thing is doing.
As for alcohol, I've never felt at all intoxicated from it, even when I've consumed close to a litre/quart. I'm making no more than a poorly-educated guess here, but I believe alcohol peaks early, would probably usually not get to more than 1-2%, and by the time you're drinking it would be about 0.2% That's largely guesswork. Sometimes I can smell the cultures getting alcoholic early in, sometimes not. I assume/guess that sometimes the yeast kicks in quickly and the bacteria takes a while to catch up, and other times the bacteria does a better job of keeping up. I've never tried drinking more than about a shot glass of the alcoholic-smelling early stage kombuchas because they're just too sweet.
Peak oil, peak ocean, peak soil, peak water... we're all going to die. Party while you can :)