How to Fix Basement Flooding Issues After Drain Problems?

Hi folks! I'm reaching out because my in-law is dealing with some serious flooding issues in their house which has a bit of a complicated drain setup. They had a garage extension built which caused some drainage headaches. The main problem is that whenever there's heavy rain, the basement floods quickly. The driveway slopes towards the garage door, which isn't very water-tight, especially when there's a lot of water coming down.

There are two big floor drains involved: one outside and one inside the garage, both leading to the city stormwater system without any pumps, so they rely solely on gravity. The original foundation's French drain connects to a small pump, but that one often doesn't handle the overflow from the main drains effectively. Recently, they discovered mold under the flooring because the floor drain overflowed repeatedly, leading to a pretty major flood event.

My in-law wants to waterproof the wall between the garage and the extension, which I think is a tough call, especially if they're considering just tiling over it. I suggested adding a no-return valve and possibly a new pump to help manage overflow, but I'm unsure how to handle the big drain in the middle of the floor with the finished flooring on top. What's the best way to address these flooding issues and make sure the walls between the rooms are watertight?