I'm looking at this part of the question :
"activating self-destruct mechanism disengage coolant system for the ship's main reactor, which then increases the core temperature and triggers explosion [by destabilising core]" is it plausible?
The kind of reactor (or other power source) isn't specified. Although fusion is suggested, it might not be fusion. Fission seems less likely. The question is generally whether ceasing cooling would ever plausibly provide a self destruct function.
I suspect the answer is "no":
Engineering and power production principles
An explosion requires uncontrolled rapid "runaway" activity. That's a "fail = unsafe" principle. Fission and raw flame can be unsafe on failure of a protective system, but generally no power source aims to be. This isn't a feature you could add to create a self destruct. It's an inherent property of an engineering/power generation method. Fission is inherently capable of runaway. Fusion (as far as we know) inherently is not - at least not on less than star-size reactors, where gravity and opacity can create runaway. Other invented systems are however you wish them.
But the coolant system can only change a runaway-capable system from running as inhibited to uninhibited modes; it can't create a runaway capable system if none exists.
Another problem is that most explosions need time to build up, however short. Even current nuclear weapons with an explosive effect (rather than just dirty material dispersion) need a very tightly fitted and insanely precise engineered thick steel case, just to prevent the components simply melting or flying apart in the first fraction of a second. Think of a gun barrel or bullet case, used to contain the propellant as it turns to gas - same idea.
A power creating reactor doesn't typically have the kind of case needed and it probably wouldn't be compatible if desired. (Because even a small space would probably be enough to adversely affect the runaway force and make maintenance impractical, although this is obviously a bit speculative and could be handwaved away).
Power production vs. power storage
To explode, you usually need to release some kind of stored energy, and this stored energy must be capable of being released very fast.
If the power system doesn't contain a store of power, or at least the ability to produce power fast enough, this makes it a bit less plausible. But if it can produce power quickly, then this still doesn't imply a store that can be released all at once. A bullet, rocket, or fission reactor contains stored energy that is designed or inherently capable of rapid release, but not all systems have this. Without much detail, this would probably be an issue to consider as well, in assessing plausibility.
On principle, engineers tend to develop power systems that, on failure of control, are inherently safe not unsafe. You wouldn't want to fly a bomb. So your car systems have several hundred detectors (including code checks) that watch for risky engine status indicators and restrict rpm if so. (What if a switch stuck, or a detector was faulty?) Ditto all other power systems. Even your toaster's power circuit has a fuse somewhere.
But in addition they design for safety even if active systems fail. If your car battery fails and the electronics lose power, you dont want the engine to be left in a full-power state mechanically with no way to turn it off because of electronic control issues.
Coolant is a subsystem. You probably won't get engineers designing it to fail, because its so critical that the cooling system can never by accident (or due to some unlikely combination of faults), fail.
Designing a bypassable safety system of this kind, with probable catastrophic effects, would be a bit like designing a lift ('elevator' in the US) for an office block where people work daily, and building in all safety features - plus an anti-terrorism feature that could raise it to the top floor at high speed and withdraw all safety controls (over cable drum rotation, lift car safety etc) and drop it at maximum speed to impact at the ground floor, just in case of a terrorist attack where the terrorists are in the lift.
In other words, however desireable a defence it might be, you just wouldn't design a safety critical system and a path that completely undermined it. You'd do it some different way.
Possible alternative solutions
If the ship had some system that accumulated immense power, that power could be released in a specific manner. That's not a coolant failure but it would meet the needs of the question.
For example, suppose the ship has weapons, and these weapons were reliant on huge power delivery accumulated over a few seconds and released in a brief but immensely powerful burst. Then the power accumulators could be used as a source of self destruct, by providing a path that would release that power against the ship itself.
You'd have to think around the issues of containment (how do you stop the power transmission system just melting instead?) and impact (how would this lead to destruction rather than damage?)
Alternatively if the ship contained something that already had explosive effect, it would only be necessary to initiate whatever explodes it. Again that's not a coolant failure. For example if it carried some kind of torpedo weapons, or mining explosives, or a store of chemical fuel needed by some kind of pods which are too small for a reactor, or was powered by immensely dense matter in some kind of futuristic force-field, then you'd just have to set off the explosives, provide a (fail-safe!) detonator for the fuel store, or release the force-field.
But as stated, and looking at cooling systems only - no.