It’s probably just the battery. There are so many electronics on newer cars that a low or dead battery will cause all kind of errors that are remedied by replacing or charging the battery so that the sensors get proper voltage.

You can also run a bottle of injector cleaner through it every few thousand miles and save the prices at the pump

Probably not, as long as you put a new clip in with the new ball joint and there’s no visible damage to the control arm.

The slowing down sounds like you may have a stuck caliper. Do you smell hot brakes after driving? Cheap brake pads often are noisy also.

Find the specs on your specific relay and test it with a ohm meter.