Well, yes. I was provisioning a new service last week and it took me half an hour of clicking buttons in AWS. Without knowing anything about Google, I would have assumed they'd overprovison compute capacity to save developer time at least for smallish requests, since they literally run their own data centres.
They do. When GP says stuff about not having flex in a cell, that essentially means "has not provisioned any quota whatsoever in that zone". Once you do the baseline work to provision some quota, generally speaking you have a somewhat over-provisioned pool to use for whatever.
More usual is "I need to run in at least three cells in region R". Thankfully, I never faced the "you need to turn up in cell EX tomorrow" without TPM support.