I’m in a very similar situation as you. The school that I am waitlisted at has orientation at the end of May, with classes starting the following week. I asked multiple questions about the process, and they helped to answer nearly these exact questions. I was told that I will not receive a denial, and that they would only be in contact with me if I were to get accepted, in which they would send me an acceptance email, with one week to make a decision. They also told me that there MAY be fluctuation in the class size, and if so, they would then determine whether or not to send out additional acceptances to those on the waitlist, but there was no way to know at that time (this was in March). Thy also informed me that in rare cases, they have accepted students from the waitlist as late as the end of the first week of classes, so for my particular school, I will know that if I have not received an offer by the end of the first week of classes, I was not accepted off of the waitlist. I hope this answers your questions, but don’t be afraid to email/call the admissions department at your potential school and ask questions! They are often very helpful and understanding of the stressful process that we go through, and are happy to help!