I think it will just create another Raid ID when you extend it that expires the following Wednesday, preserving last week's progress.

AKA: Only the person extending gets saved, and you have to join their raid to get saved again.

And extension is limited to one week only.

it is most likely that only the raid leader can extend the raid

From what i have seen the raid lockout extension is on a personal basis. so no one can extend it for you

Each person extends their own raid lockout. A person who doesn't will have a new raid ID the next time they step into the instance. A person who extends will step into their old instance.

The only way to get "locked out" of being able to remove the extended lockout is if you do something during the extension that would have you saved. Ie you down a boss. If you wipe and wipe and wipe and then remove the lockout, you can start merrily from the beginning instead. If you down the boss, that's your instance ID for the week.

If your raid leader extends his lockout but the rest of the raid doesn't, they'll enter the instance and see the timer ticking down asking if they want to be saved. If someone not the raid leader extends their lockout but nobody else does, they'll be standing alone in the starting area and wonder where everyone is.