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Is it correct that the ETA is not needed?
Yes. ETA is required only of air passengers.
Am I right to assume these are just as valid when re-entering the US at the US/Canada border?
Yes. You don't actually need an I-94 when arriving at the US border, but having one may make your entry smoother. It's common for the I-94 to be reused if you haven't traveled beyond "contiguous territory," which you will not have done. "Contiguous territory" comprises both Mexico and Canada.
If for some reason they decide you need a new I-94, it will only cost $6 per person.