I suggest you contact firstname.lastname@example.org.
The forum is a platform for the community to support each other. If you need answers from Mendix, better contact Mendix directly.
Once per registration and no gap between exams. For rapid developer you have unlimited time per question and for completing the entire test. If you have been using Mendix for a couple of weeks and have done the rapid courses available on the academy.mendix.com you are likely to succeed.
Anyway: good luck!