RE: Shocking
Very complicated for a volume compensation.
Much easier is.
1)
Take a CLOSED celrubber.
These are all small captured air bubbles.
The celrubber with its captured air can compress when the shaft is going in.
Look at the elcon shocks.
Simple and very effective.
Located in the top it will always float upwards and can never get jammed between piston and cilinder wall.
No need to fix it even as it will always float in the oil and stays in the top (unless your car is upside down..).
2)
Take a simple rubber hat like fg has.
Mount it upside down with air above so it can be compressed as well.
Bit more vonerable then option 1 as now air might be possible to slip through and get into the oil medium where option 1 will prevent this at all time.
3)
double piston which seperates air and oil.
You can add even a small spring between the top and this seperating piston.
It gives the option to easily change this small spring for a stiffer or softer one, to play a bit more with the dampening characteristics. Basicly the same as the cad shockers.
As an extra you could drill a hole in the top cap and mount a pneumatic restrictor, then you can play from the outside even more with the dampening.
Key is to make a good seperation of the air/oil.
There are even more complicated options possible but I think this is beyond what a scale model needs.
Been there, done that, tested each option even with pre-pressurized compartiments.
I would opt for no. 1. and would first focus on closing your ingoing piston rod.
Other things are all fairly easy to solve / addept.
The closing of the constant in/outgoing movement together with the dirt that will help to damage the seal will be your main concern.