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903.12. If a commander would be put into the exile zone from anywhere, its owner may put it into the command zone instead.
If the owner chooses to let a commander go into the graveyard with the expectation of returning it, but the return fails, can the owner then move the commander to the command zone so it can be cast from the command zone? Or does the choice to move the commander to the command zone need to be made immediately?
No. The rule you quote (and its equivalent for graveyards) is the appropriate one and mentions nothing about delays. The choice must be made immediately, but can be remade whenever the commander changes zones. For example, if you allow your commander to go to the graveyard, and then a card is played to exile your graveyard, you can once again choose to send your commander to the command zone instead. Once your commander has arrived somewhere (exile, graveyard), it stays there until a card tells it to move.
For completeness, the graveyard rule:
903.11. If a commander would be put into its owner’s graveyard from anywhere, that player may put it into the command zone instead.
You quoted the wrong rule, but the answer two both situations is the same.
903.11. If a commander would be put into its owner’s graveyard from anywhere, that player may put it into the command zone instead.
Whenever you see the word "instead", you should understand that this is a replacement effect. If the owner chooses to let the commander go to the graveyard, he is giving up his option to move it to the Command Zone.
614.5. A replacement effect doesn’t invoke itself repeatedly; it gets only one opportunity to affect an event or any modified events that may replace it.
