Oh, and that fusion engine is clearly stalling out due to overheating from how coked up it is. A very 20th century problem repeating over again in ... whatever future century this is.

So yeah, they clearly need to run some kind of deep cleaning cycle and then resign themselves to using better quality fuel, as well as doing something about the mixture of reactants as it's very much out of balance.

Probably all down to whatever neo-dystopian non-unionised spacer company they work for being too cheap to spring for a good supply of helium-3 or an interstellar hydrogen collector matrix as they've found it's far more economic to just have a basic scrub-out every few thousand light years at whatever starport they can limp into, whilst feeding whatever crap they have available into it the rest of the time (non-recyclable organic wastes, mining slag, etc), making it the last few billion miles when it's finally too gummed up to host a self-sustaining reaction by running on the slightly better quality fuel ("easy start" aerosol, if you like) of Expendable Conscripts.

The switch inside the reactor being there so that no-one outside the reactor can prank the cleaning crew (and massively annoying the company THEY work for) by "accidentally" restarting it whilst they're in there. The system in control of kickstarting the fusion reaction from some bundle of low-ish atomic weight matter being designed to override any exterior commands if there's a human inside, UNLESS they assent to being used as emergency fuel by pressing the internal primer switch... can of course just leave a coffee cup on the command console "start" switch if you're the only engineer left, so once the primer has been pressed in for the customary four seconds (like forcing a PC to power off), it initiates reaction without any further active intervention.

Failsafes are all well and good, but you still need some way of overriding them and doing something otherwise dangerous, deadly, or otherwise damaging in an emergency situation where the alternative is a bigger disaster.
(Which is why I'd have a hard time accepting autonomous vehicles without *any* manual control option - what if the road starts giving way due to a landslip and your best option is to steer off into the bushes, as the hillside above doesn't appear to be moving yet? Or stopping one that's glitched out and running out of control requires speeding ahead and then deliberately crashing both vehicles against a barrier to slow the errant one to a halt - something that was actually in the news recently? There's too many situations where human ingenuity would still be needed, and any synthetic copy of that would be just as falliable as the real thing, so why bother going to all that effort when you can just use the real thing?)

Though of course in this case, the disaster is "falling out of hyperspace because the main drive reactor has stalled from neglect and misfuelling, and the captain being unwilling to requisition the cost of 90 minutes of tug time (30 out, 60 back) from head office", because of the ultracapitalist, life devaluing culture pervading this bit of the galaxy.

Ah, the dangers of the singularity, hard AI, etc.