You can be charged with what is called “DUI less safe”.  In essence, the state will allege that, while not over the legal limit, you were a less safe driver and had alcohol in your system.   Weaving within your lane of travel and speeding are both examples of less safe acts that can form the basis for a DUI charge.

Immediately.  The officer will likely provide you with a 1205 notice of intent to suspend your license.  You have a limited time frame within which to notify the state that you want a hearing.  If you don’t do so, your license will be suspended.

Often people get confused, because they submit to the portable breath test on the side of the road.  That test result is only useful in the sense that it tells the officer whether you have alcohol in your system.  The number generated is not admissible.  You absolutely can refuse to submit to that test.  The test that you are given at the police station, the intox test, is different.  The number that it generates is admissible, and, if you refuse to take it, your license can be suspended for twelve months.  Ultimately, it is a balancing decision that you have to make.  If you take the test, the results could be damning.  If you don’t you could lose your license.

No.  Roadside field sobriety tests are voluntary.