First of all 75% is not cast in stone, it is just a rule of thumb or a matter of taste.
The VFF is your direct input, it bypasses the PID controller. Please read this pdf
Some pilots fly 700 class and bigger without FB and without FBL-unit etc. , these pilots don’t need/want PID controllers of any kind, this translates into PID=0 and FF=100%. AFAIK nobody is flying a 450 size without FB or FBL-unit. You can fly every helicopter with FF=0 if you have PID>0.
NO, I start tuning by setting FF to have about 75% of the needed cyclic, but you can use the default value as well. This gives me the necessary control (I use Dual Rates and Expo as well) and it allows me to start with low(=maybe not enough) PID values to avoid oscillation drama.
NO, usually you set the cyclic to have about 6° to 8°, The FF should or could be set to give you 4.5° to 6°.
Use the default values, lower ACCEL_Z_P from 0.5 to 0.3( and tune that later) and start the usual tuning process. I use Acro others prefer Stabilize.