The unobstructed open air balcony is very hot until noon, which large leaved plants should be plante

It's hard to have sunshine! Unless the sun comes out from the north!

No one is at fault. However, compensation is required. There is a law called no liability compensation, that is, whoever is the user of the balcony needs to be responsible for repairs, otherwise, the user of the balcony must be responsible for compensating for all losses caused by water leakage downstairs.

As long as the property allows, you can make the terrace into a sunroom.

It turns out that there has been SBS waterproofing membrane under the cement protective layer of the balcony? If not, then look at whether there are cracks in the ground, you can use high-pressure grouting polyurethane, and then make a two-layer Henkel waterproof coating on the surface, and then paste tiles.

It's best to cover the spire with a casserole, remembering that the casserole should drill a small hole in the bottom of the pot.