1/ it’s hard to say… it looks like your network is quite clean, interference-wise, so by changing the HO Qual threshold, you will merely reduce the number of HO, while the voice quality remains ok.
As you said, what you did didn’t change much the KPIs. But reducing the number of HO is a good thing. In a way, you did improve one KPI that you were not monitoring (the number of HO).
The only thing you must ensure is that the voice quality is not degraded.
2/ The best way to reduce interference is to locally modify your frequency planning, then use AMR, power control, dtx, etc.
changing the ho thresholds is just hiding the problem, which will sooner or later re-appear.