One other thought - in terms of clearances you can always refuse a clearance if you are not able to comply and indeed ATC are often aware that you may not be able to comply. For example you will often hear the controller in class D airspace ask if you need to maintain VFR. In any event a clearance, regardless of the plan filed, is best thought of as a negotiated contract between you and the controller, it is requested by you, accepted or denied by the controller, and the terms of that clearance then confirmed or re-negotiated by you.