Scenario 2, part B and C : That’s VNav doing what it’s supposed to do. Either turn it off, or clear the constraint. Use VS or FLC.
Scenario 3. Easiest to use FLC at high altitudes, then VS to manage the descent profile accurately at lower altitude.
Remember VNav on the 650 is a “modifier”, not a mode like it is on a Boeing. So with VNAV turned on you get VFLC and VVS. Without it you have FLC and VS. The only difference is VFLC and VVS will obey any magenta altitude constraints shown on the PFD, as well as the blue altitude constraint.
Hope that makes sense?