How can I use a List of Option Process Variables on an Instanciation form
Process variables are not the good option. Each case will create a copy of them and store them in db (for each instance). And as you did not started the process (you are in the instantiation form)you do not have the data.
There are different ways, but the one I prefered for static data as this one was to create a rest api extension that return these lists (potentially you will have more than one).
Something like ../API/extension/masterData?q=status
that returns a list of status