Lucky you with no ethanol in fuel, I'd kill to get some. Do they even use any ethanol in the fuel there at all? If so, the stuff may be leaching into the fuel lines for non if like here. News reports of regular fuel buyers getting premium if the last guy before you got it, some stays present in the line system to come out with the next fill even though switched to another type. The premium users here b-tch when they are not getting all they pay for. And with the help we get now there is no guarantee the tanker guy is putting the right fuel in the right station tank. Premium being often made using ethanol now makes even the fuel supplier suspect too, they get it cheaper than the gas itself due to the subsidy on it. And the mileage drops making it that way so they sell more too.
If the seats are good the drain plugs will seal by themselves, the o-ring then needed to seal from bowl leaking on top of motor when the drain is opened up. It also provides backup seal if the drain seat leaks a bit.
Yes, let it sit. I have rebuilt carbs before to have them leak with pretty fresh o-rings, let bike sit one day to let the fuel get to the rings to swell them and everything fine and never leaked another drop. ALL rubber parts swell a bit when the fuel hits them, when they shrink from drying out; if you do not disturb them and they do not shrink enough to crack often they will swell back up to be fine.
A LOT of people have complained as well about the mostly Chinese carb replacement parts we get, the fuel needles often have issues with swelling due to a lesser grade of rubber tip being used. Even normal non-eth fuel can make them act up if they are not original, and if they ARE original then I would expect them again to act up due to age.