I don't understand clearly your question , but in my opinion Fz is constant and scaling factor are not used in all the pacejka models , there are some models without these parameters , i need to search this further more in the meanwhile you can check these links maybe some help :

http://www.control.lth.se/documents/2006/jsve_7614_ScaleF.pdf      

http://pierre.lemerle.free.fr/site/pl-tirefit/pl-tirefit.html 

In  the Paceka model you calculate the lateral forces as a function of normal force (Fz) and side slip angle (alpha). Of course the normal forces vary and thus the lateral forces.

Fz is surely constant when the car is standing still.

I mean that in the following equations there is no effect of vehicle load distribution and acceleration.

I think you have to calculate the load distribution yourself. also you can use a simpler magic formula.

first you need to search for the wheel nominal load Fz0 if you didn't find it search for the maximal wheel load ( the wheel that you are working on ) , then use some kind of percentage from your choice in general 80% of the maximal wheel load if good enough so Fz0=Fzmax*80% for exemple ... i hope this helps inchallah .

Fz is one of input parameter for MF tire model. During the driving, the value of Fz for each wheel is changing. You need to get the Fz for each wheel using estimation. Fz is includes the static part and dynamics part which relates to lateral accel., longitudinal accel., and roll motion...  When Fz changes, the tire forces are changed accordingly.

You may refer to my paper for some ideas

https://www.researchgate.net/publication/245391027_Design_of_a_vehicle_lateral_stability_control_system_via_a_fuzzy_logic_control_approach

Article Design of a vehicle lateral stability control system via a f...

The work http://alturl.com/shn64 and its references may help you to understand your questions better...