Numbers please go back and read the thread where all this was gone over and by experts. Yes a that point other tests were terminated. Please provide other samples where "too low" was the reading and she continued.
The knife IIRC had more amplification than other samples. While other tests were ended earlier she didn't give up on the knife until she found a profile. Had the same methodology been applied to other samples IIRC the speculation was that they too might have produced profiles.
I think the point the lab savvy posters here have been making, as well the independent experts in the real world, is that Stef wasn't "finding DNA profiles" in the sense of getting reliable results. Rather she was essentially manufacturing a result and then trying to palm it off as legitimate. Something she could only do by hiding the underlying lab data that tracked how she obtained those results and the presence or absence of contamination.
Diocletus, Chris, Planigale, or Numbers could probably be more articulate here.
And yes, Mignini needed to solve the case because foreign students were fleeing Perugia, and taking a fair share of the local economy with them, under the glare of an international media circus. Plus Mignini himself had just been indicted for abuse of office, having claimed Mario Spezi was part of a satanic sect that had been responsible for the Monster of Florence serial killings, and whose purpose was to obtain the female sex organs from those sex mutilation murders for use in 'sacrificial rites'.
So Yes, Mignini needed to solve the case, and finding a satanic rites orgy murder conspiracy would do wonders for his situation at the time.
You could do a dissertation on what Mignini actually believed or would be able to admit to himself about what he was actually doing. I'll leave that speculation to experts.
But I must sadly lean towards Rose Monty's take on the attitude of the police. A conviction is a conviction, its not their fault if the court agrees.