I cannot comment on the bulk of what you ask, I'm sure Flowman et al will be along soon, however from the radar perspective, I wouldn't expect on getting anything better once airborne. Some ATC providers are instructing their controllers to adhere to the flightplan religiously, especially where a difference of requested and filed levels occurs. HOWEVER, there are certain routes I know of in the UK that only become available after certain times due to danger area activity. Should notification come that the activity has ceased early then the sector responsible for the route would normally offer the now available shorter route.