Couldn’t they have just attracted the ringwraiths to an ambush? I know the witch king is supposed to be unkillable, but there’s 8 others…

The witch king was absolutely not unkillable, just hard to kill. Eowyn slew him after Merry stabbed him in the leg with a barrow blade. There is no indication that, had Frodo and Sam failed, the witch king would have risen again.

If I recall correctly, it had been prophesied that he could not be killed by a man, which the people in-setting took to mean he couldn’t be killed by a person. It’s only after he died that people realized it meant he’d be killed by a woman.