1. Can I do the (written and oral) selection tests on another date than the date proposed?

No, the written test is on a set period of days. 
For the oral test at Enabel's (part 1: motivation interview, and part 2: group exercise and  competence-oriented interview), candidates can choose from among several dates (via Doodle) that have been e-mailed to them. The candidates can choose the date that suits them best.