What you need for heating also depends very much on the temperature of the room you're in. Without that, no one can tell you whether you need more or not.

But yes, if your cool side gets too cool even when your hot side is at the right temperature, then you either need an additional heat source or you need to insulate the cage. You can make a pretty big difference in a glass tank by putting foam insulation board around three sides of it plus whatever parts of the screen top are not occupied with lamp fixtures. That alone might even make for enough less of a temperature dropoff from one side to the other that you don't need another heat source.

If you do need another heat source, a CHE will work. If in doubt, get a higher wattage one and the thermostat will dial it down as needed. Just make sure the fixture can handle whatever wattage you get! For overhead heat sources, I like to leave the thermostat probe dangling down so that it can't be sat on or peed on, which would throw it off. Since the probe will likely be a bit off to the side and not directly under the CHE, you might need to set it for a somewhat lower temperature than what you actually want under the CHE. Just check it several times in several places to figure out exactly how you need to set it to get what you want.

Re. basking: First of all, "crepuscular" is not the same as nocturnal. Crepuscular means most active around dawn and dusk, nocturnal means most active during full dark.
But also, ball pythons do occasionally bask during the day, both in captivity and in the wild, either openly or cryptically. Cryptic basking means the animal is mostly hidden, but some parts of the body are sticking out into the light. They're maybe more likely to bask if they have basking areas with visual cover, so they can still feel somewhat hidden from view. So providing a basking area with branches or leaves (fake is fine) or walls or something near and around it might make it more attractive.

They don't require it for survival the way many lizards do, but it's not correct to say they never do it.