Drilling holes in cast iron planter

Plant in a container with drainage holes and place inside the other container.Make sure you empty it on a regular basis

I would say probably not, but you can place an Inch or two of pebbles in the bottom to keep your plant's feet from getting soaked, and/or you can use a pot with drainage holes that fits inside the cast iron planter

You should be able to use a metal cutting drill bit and not damage the planter. Use a little oil of any kind to lubricate and cool the drill bit.

Dont know the size of container but I have seen empty plastic water bottles used as filler for larger containers.

If you can’t drill holes, put about an inch of rocks so your plants won’t rot. Or plant some water plants in it.

I would add pebbles either way. May need to coat it with paint to prevent rust. Put high humidity needs plant!

I once had a cast iron plant stand, I melted paraffin to put in the bottoms of the plant holders. But if yours is a single unit, the stones would be a good idea and maybe some sort of base for them that is impregnable by water as eventually the iron would corrode.

Put a planter inside the planter