Are the mining incentives of bitcoin compatible with the existence of a competing Proof-of-Work chain?
Both chain in your scenarios can coexist because the current value of a coin (and the current mining reward) isn't the only factor miners are taking into consideration when they have the choice to mine two different coins.

They also put their personal conviction into consideration when choosing what to mine. If they prefer the change brought in chain A, why would they mine chain B that doesn't have it ? (Mining chain B in case of a HF would be being against chain A in that case)

So yeah both chain A &amp; B can coexist as long as the differences between them are big enough. If not their would be no incentive to choose to mine the chain with the least value.