People have participated in Onboarding Week as early as their first week and as late as 6 months. It is a valuable experience at any point, but around 2 to 3 months appears to be the sweet spot. It's enough time to have been immersed in the Cloud Foundry domain but not so far that it becomes more of a "continuing education" experience than an "onboarding".
If I knew, I promise I'd tell you. Resources are allocated in an inscrutably arbitrary fashion, such that some Onboarders will hit many quota walls while others have no problems at all. You have the option of requesting quota increases for all GCP accounts before allocating them to pairs, but since it is a problem they're likely to hit in the wild, I tend to let them troubleshoot it themselves.
No, please don't. This mistake costs the company a lot of money.