You can plant the agave into a large black plastic container, and place it into the urn and cover the surface with a mulch for the warm season, then take the plant + plastic pot out if the urn for the winter.

I don't know how to safely get rid of the ants, but if you can figure out some way to bury the pot into water for a few hours to drown the bugs, that would work very well. It wouldn't hurt the plant. Any chance you can submerge the pot into a large garbage can maybe?

Often the presence of ants means there are mealy bugs and/or aphids around in large numbers, so check to make sure there aren't other critters hiding in the plant. Try to minimize the amount of "organic" components in the soil mix, which will reduce the likelihood of worms, isopods and other normal inhabitants of garden soil. A high % of perlite would be nice as it would make the pot much lighter and easier to move around.

The cat thing? Never heard of that before, bizarre.