The Density parameter is setting the rate of the grain. When you have TUNE Key tracking activated for the Engine, and if you set the Granular modules random Grain Pitch parameter to zero, then each grain will be determined by the played notes.
(The Pitch Bend can be used to bend the pitch +-3 Octaves.)
But too short grains will not be long enough to determine the excact pitch.
If you in your example deselect pitch Key tracking for the Engine in the TUNE section so the keys does'nt have any pitch effect, and also set both the Start and the Pitch parameters in the Granular module to Zero, then one will only hear the pitch effect the Grain Density/ Rate apply. If you set the Density parameter to 220 Hz, then you will get a A3 and 100 Hz about A2. But i'm not sure other rates will match.
Yes - You will only have the pitch range the Density parameter allow, when it's changed.
The Grain size have an impact on the sound.
I don't know what Halion excactly does, if it's different from this. Is there an example in a video somewhere?
Is the difference the Grain Density/ Rate range only? I can only see it has a percentage speed setting, if that the Density/ Rate. It's not in Hz.
It also have something with Zone pitch and such, but i don't know what that section does. I know nothing about Halion. I don't have it.