Basement floor drain / sewer drain vs. sump pit / pump?

I'm a new homeowner and am still learning the basics. Our basement occasionally has water issues when it rains so we are installing french drains. We already have a pit which has a sewer drain at the bottom. Are there any downsides to removing the sewer drain cap so the water can escape? If I do this, do I still need a sump pump or this unnecessary? If I do need a sump pump, should I also get a backup sump pump (in case of a power outage) or is this going overboard?

If I uncap the sewer drain, do I need to install a sewer backflow preventer or is this unnecessary as there have never been any reports of backflow issues in my town?

How do you know the "drain" is connected to the sewer? Are you sure the drain is trapped?

In many places it is illegal to drain groundwater to a sewer.