1) Why is there a hashMerkleRoot mismatch?
For some reason, this block, block 246159 (according to blockchain.info) is failing validation. Can't really say why. My guess would be that somehow it got corrupted in that file and it is missing some part of the block so the validation fails. This is probably where it gets hung.
2) Why is 71.93.44.219 misbehaving?
3) Why should bitcoin-qt CARE about some node misbehaving when it is allegedly NOT downloading blocks, but simply rebuilding the database on the blocks it has?
Bitcoin Core is still connected to its peers. Each peer receives a banscore based on whether the node thinks it is misbehaving. After the score reaches a certain threshold (default 100), the node should disconncect from the node. It cares whether the node is misbehaving since the misbehaviour involves spam and can eat up your bandwidth.
4) Obvious follow-on... is it, in fact, despite what you said, in *fact* redownloading those blocks after all?
It should still be downloading blocks, just not ones that it already has. The node is constantly receiving and sending data, but it should not be redownloading those blocks, only downloading blocks that it still does not have.
5) After the first 10000 times* of printing this little set of error messages, should not the client somehow grasp that what it is doing is NOT WORKING and, just perhaps, try another node? Or even give up and say "There is something wrong, I'm stopping now."?
*obvious snark - I know what computers can and cannot do. I suppose my snark is really directed at all these coders who must somehow have never seen this behaviour before and never thought to put a little 'break' in that loop.
It should, but I don't know if it actually does (probably not).
Your best bet to fix this would be to just let it completely redownload and reindex the entire blockchain. I know that can be frustrating and time consuming, but it should fix the issues. If that doesn't work, try reinstalling Bitcoin Core.
knightdk is just working on his required activity for the signature campaign.
At least I'm being useful to someone, unlike most people in sig campaigns.