I don't believe you could save the emissions from animal agriculture.

The easiest is to stop converting more land to animals use.

Most animal ag is on land that isn't suitable for growing crops,

But 6% of global emissions are from feeding crops to animals.

if it was ended, the land would become useless

Aside from reducing our emissions by 16%, meaning it would be about as useful as removing all emissions from the transport sector.

and left to go wild where it would support just as many just as polluting animals,

Laughable, but if if it where true it could easily be solved with reintroducing predators and rewilding the artificial grass lands.

but with no possibility of treating the pollution problem as no one would be managing the wild animals

What is more polluting? Animals shiting in the forrest every day at a different place or month worth of stored manure deposited on one field in a day, with barely any plant at the moment? Which of those will naturally break down and which will be washed into the ground water?

Where are the huge amounts of biomass fermenting into methane? Some swamps and every farm.