They did do something about when they keeping sitting on the final component needed to make it work for centuries every time they invade the galaxy.
Remember with out the Catalyst aka the Citadel the Crucible is useless, and the standard Reaper invasion starts by taking the Citadel in a massive surprise attack while also destroying the galactic government followed by hunting down and exterminating whats left of the galaxy's civilization.
The only reason they didn't do that until later in Mass Effect 3 was that the Citadel relay was taken out of play by the Protheans, and then later Shepard killing Sovereign.
So they really don't have to sabotage the plans since their usual invasion plan makes finishing the device impossible.
And its not like they wouldn't notice that the plans call of the use of something that has the same name as the AI controlling them.
Besides an "unconventional refuse option" sounds like wishful thinking about a magical deus ex machina that can be pulled out at the 11th hour so as to get by fact the the Reapers can't be beaten without some ultimate weapon one of the previous cycles may have come up with that the games previously hinted at.
Basic risk management. They used their standard invasion MO in the previous cycle and the result was that the Protheans managed to build the device and nearly deploy it. At that point why would you possibly leave to chance the possibility of the next race taking things further and deploying it? The Reapers effectively ended up sitting on the final component in this cycle but that didn't prevent the alliance from building the Crucible, deploying it, and actually delivering it. Something that would have been rendered meaningless had they either sabotaged the plans or developed countermeasures. Again, you can claim arrogance on the part of the Reapers, but considering the incremental positive progression of the plans through the cycles it was inevitable that eventually someone would build it and use it. That goes beyond arrogance and strays into stupid.
Again, comes back to risk management - given that the last cycle were close to deploying the weapon, was it safer for them to do nothing on the assumption that their plan would still work, or would you put mitigating plans in place to reduce the risk? Mitigating plans by the way which would likely have required little effort on their part. They'd just wiped out galactic civilisation, were they really THAT desperate to get back to dark space for their millennia-long nap?
Re unconventional refuse, they needed the Catalyst because the Catalyst was part of the Citadel. They needed the Citadel because it was a huge source of power. Find another source of power. Or find a way of rendering the Catalyst irrelevant in being able to harness the Citadel's power. You found the Leviathans FFS, they've been around even longer than the Catalyst and the Reapers. They're the original apex race and they claim to be technologically superior to the Reapers, yet they have nothing to contribute to this end?
The whole ending shambles is incredibly illogical. Common sense seems to go out of the window on all sides in a last ditch battle of the dumbest.