utACK, apart from the above nits.

Added commit to correct nits (plus fix a missing include).

ping

Rebased for clarity after #5170 being merged.
Nit fixes squashed too.

Needed rebase.

This definitely makes the code more readable, it rebases on 0.10 without problems and its the sort of change that helps to isolate and track pow related operations. It adds the following functions to pow.h:

bool CheckChallenge(const CBlockHeader& block, const CBlockIndex& indexLast);
void ResetChallenge(CBlockHeader& block, const CBlockIndex& indexLast);

/** Avoid using these functions when possible */
double GetChallengeDifficulty(const CBlockIndex* blockindex);
std::string GetChallengeStr(const CBlockIndex& block);
std::string GetChallengeStrHex(const CBlockIndex& block);
uint32_t GetNonce(const CBlockHeader& block);
void SetNonce(CBlockHeader& block, uint32_t nNonce);

It may be desirable to add to the comment: brief reason why to avoid those functions and a mention where they are currently called (rpcblockchain.cpp and rpcmining.cpp).

NACK

"more readable" is not a good justification to be messing around with this code. If anything, I'd argue it makes the code less readable as what it's actually doing has been obscured by more layers of abstraction.

Well, it introduces the Challenge term on check/reset block-index operations which seems more readable to me compared to the if checks. It also does so keeping block as first argument for all new functions, consistent with other existing calls like CheckBlockHeader. At last it introduces get/set function wrappers to operations accessing the nonce. I think this is an improvement and should be merged. I'm not sure what do you mean by abstraction: that has a particular meaning in C++ and I see no real abstraction introduced and even on the linguistic level the function names are good explanations for what they do.

Closing for now, at least until #5599 has been merged (if it gets merged) and I can propose something more complete.
For those interested, this is the patch I intend to maintain permanently (to make sure nNonce and nBits are not used directly out of pow.o): https://github.com/jtimon/bitcoin/tree/noproof