The new point you added about what will happen when the block reward goes from 25 to 12.5 is very interesting and I hadn't thought about that previously, but it bears consideration especially if you bring it to its logical conclusion: a share becomes worthless when the block reward becomes 0.  The payout logic remains the same, (block reward + transaction fees) worth of shares are paid out, but the intrinsic "value" of the share, which previously was always some positive value in BTC, becomes zero.