An ER will not get you into rehab. They may get you into Detox for a few days, but that is much different than rehab.
Your blood alcohol will not drop .20 in an hour, you normally metabolize 1 drink per hour. It takes a lot more than 1 drink to get your blood alcohol to .20, which is 2-1/2 times the legal driving limit in all 50 states of the USA.