According to ICAO Doc 4444 : Air Traffic Management (PANS-ATM) all the contigencies mentioned above are stated on chapter 15 " Special procedures for inflight contigencies in Oceanic airspace"
I will not re write the whole procedures again but
Under 15.2.2 : general procedures ... Where you have to climb or descent 500 ft
Under 15.2.3 weather deviation procedures :....where you have to climb or descent 300 feet depend flying east or west
Under 15.2.3.3 : actions to be taken if atc clearance cannot be obtained
All of these procedures come under chapter 15 "special procedures for inflight contigencies in OCEANIC airspace"
I's written the same way on lido
Be careful this is for OCEANIC and It is not applicable to Europe RVSM .
As for level deviation, 300 ft is considered a level bust