[onedoug]

As for its outgassing as it approaches the Sun - for perhaps the millionth-plus time - must it somehow be replenished by some water source in the outer solar system?

[Swordmaker]

As for its outgassing as it approaches the Sun - for perhaps the millionth-plus time - must it somehow be replenished by some water source in the outer solar system?

There is no known model for accreting new water or ice in the outer orbit. The gravity acceleration of 67P is only 1.5cm per second per second. The escape velocity is a very slow walk.