A quick test will tell you whether the alternator is sending charge to the battery. If it isn’t, then you need to start checking the wiring and things like the whether the car is earthed properly. Perhaps there’s a blown fuse of circuit breaker that is preventing the flow of charge from the alternator to the battery.

If the alternator is, in fact, charging the battery when the engine is running, then you’re looking for a problem such as an aftermarket alarm or a boot light or some other electrical gremlin that is draining the battery overnight. It would also be worth checking that the new battery and alternator for correct operation and capacity. It wouldn’t be the first time a brand-new component has been faulty, fitted to a car and then confounded the mechanic responsible.

The clicking noise is probably the central locking actuators that physically lock and unlock the doors. If they’re constantly cycling, it won’t be long before they fail. Which sounds like why the car locked you in.

The reason they’re cycling constantly is probably something to do with either the sensors that tell the car a door is ajar, or the body computer that controls a huge array of functions in a modern car. An auto electrician is probably your best bet to have this sorted.

Yes it is, which means that if you don’t do regular runs at highway speeds, the filter is prone to clogging up and requiring either cleaning out or even replacement. The filters for this car are advertised for anything from $200 to $800 for a second-hand DPF, but you could be buying one that is well past its best.

In a more general sense, any second-hand Epica would need to be super cheap before you’d consider it, as they were a slow seller and not a particularly good quality car even when brand new. Unloved then, they’re unwanted now. There are definitely better choices.