Hi max, trying to answer your three points:
1) what do you mean with "elementary product flow"? In LCA one can dinstinguish product, waste and elementary flows. I have never heard of a elementary product flow.
After creation of a new product flow object in openLCA, no environmental impacts will be linked to this flow. This is done via modelling the life cycle and thus linking your new product flow already existing flows.
2) system processes aggregate the pre-chain, making them a bit intransparent. The impact result should be the same.
3) Please only do changes to database processes after you have copied them. This way, you ensure, that your changes only apply to your modelling efforts and are not used in the background for the whole database.
If you manually change 1 -> 500 m³ and the process is not parametrized, you also have to manually change all the other flow amounts.
But why would you even want a linear multiplication with 500?
If you need 500 m³ you could specify so in the process, which uses "excavation, hydraulic digger" as an input? This way all the flow informaton of "excavation, hydraulic digger" will be multiplied by 500