The fuel lines aren't plug-and-play rubber hoses—they're cleanroom-welded exotic metals that pipe toxic, explosive fuels that corrode (and can explode on contact with) most materials. Remember the SpaceX Dragon craft that blew up on a test stand? That was a failure of their analogous subsystem–hypergolic oxidizer ignited a valve, which was machined from solid titanium, and exploded.
Just an aside, but im amazed they can even reliably pinpoint the root cause for these explosions. How do they do this? Some mixture of live sensor data and just general intuition (oh the explosion started here and we know x is a limiting factor, etc)?.
They have a lot of sensors on everything, plus they'd be filming from a lot of angles, and the additional benefit of ground testing is that they can look at the debris. You can piece together where the explosion happened based on the charring and where the parts broke and how.
If I recall correctly, the Dragon explosion was especially interesting because the reaction was previously unknown. So, it wasn't just a design flaw that allowed NTO to leak into a helium line, it was also a new discovery that titanium can react with NTO under high pressure and ignite.
The spacecraft's already built with a (large) surplus of thrusters—the extra thrusters are all right there, pre-attached. It's because of that safety margin that NASA relaxed their requirements and launched with thrusters exhibiting high failure rates in testing.
Redundancies don't help you if you have a safety culture that treats redundancies as consumables. The more one team widens a safety margin in one place, the more another leans on that safety margin, and relaxes their own.
I think you'll find that they have more thrusters on the spacecraft that they actually need - they can control attitude even if they lose one or more thrusters. So the possibility of the failure of them is already in the design.
