Hey 9083322,
1. If i am not mistaken there is a 21 day limit between 2 In person GMAT exams. Not sure about the Online version of it but if it did you can assume a max of 21 days
2. In my experience the schools do get to know that you had an attempt but they wouldn't know the exact score and details of the same
3. With the online GMAT, unless you send the score to the schools, they will not know anything about your score and it wouldn't even appear in your attempt history
4. The interface is a bit different than the usual in-person GMAT registration so as you proceed further in the GMAT Online registration link you will get to chose the date and time of your exam. You will have to then make the payment
Hope it answers your questions
Kudos if it helps!