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Regarding b), I believe it's the MPL order you are referring to.

http://usequities.nyx.com/markets/nyse-arca-equities/order-t...

Some dark pools also allow trades at subpenny increments, although of course (being a darkpool) the orders are not displayed.

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Transactions that occur in dark pools (over half the liquidity on NYSE listed stocks these days) typically get posted to the exchange after the fact. This used to cause anonymous hedge fund I used to work for all kinds of problems in the leas liquid markets as half the daily volume would suddenly post dive minutes

This seems inaccurate. Dark pools make up 15% of market volume.

Midpoint orders exist on all the venues. All but EDGA/EDGX provide some rebate for resting a midpoint order. DirectEdge charges both sides for what they refer to as "midpoint match".

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