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So even with 100 octane fuel being the common fuel in January 1942, the "All out" limit remained +6 1/4. So obviously there is no relation between introduction of 100 octane fuel and the fact that the "All out" limit remained +6 1/4.
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Yes!!
Read the instructions for using +12lbs boost. First you have to override the normal controls and it emphasized that it is a very overloaded condition.
+12lbs is almost 3 times more pressure than the engine was designed to handle.
The modifications to the cylinder heads changes the frequency harmonics in order to reduce the incidence of cylinder head cracking. It does not increase the design strength of the engine.
BMW did the same thing when attempting to raise the motor to 1.8ata in the BMW801D2. The service trials resulted in a high incidence of cylinder cracks so they changed the cylinder barrel liners to ones that conformed harmonically under the new load.
Think of a tuning fork, if you want to change the frequency you adjust the length of the tines. To change the frequency in the merlin III, they added .020 inches to the spigot.