Note that there will not be as long a period for early voting as there was for the Super Tuesday primary. The first day to "early-vote" is Monday 16 May, and the final day is that Friday the 20th. Considering how light turnout typically is for any runoff, there is no reason to keep the early voting locations open for twelve days, only to have about twelve people show up.
To vote in the runoff, if you are not already registered, the registration deadline is Monday 25 April. Yes, you may vote in the runoff if you missed the primary.
Meanwhile, Greens and Libertarians will have all their state and local nominations taken care of by mid-April. Both parties will hold their state conventions in Greater San Antonio the weekend of 9 April, with the Libertarians getting started on Friday the 8th. The Libertarian Party's National Convention takes place in Orlando over Memorial Day weekend.