Unfortunately won't work unless you flood the world, because Mormons only do baptism by full immersion.

They only need to wait a century or two then... and try to release more greenhouse gases if possible.

Even discounting the procedural difficulties, which would make such a baptism invalid to the church, it doesn't count without knowledge and consent.

How do the dead ancestors give consent in the current scheme?

Asking them in the afterlife, apparently.

Correct. Proxy baptisms are never considered to make someone a member of the Church, precisely because it is not possible for the earthly Church to confirm with a dead person that they have given their consent. It is merely considered to make the opportunity for giving consent available to the person in the afterlife.