Yes, I would point TRS22 to its original installed local data folder. You can still add whatever you want to that folder.
In my case I have 3 executables on my main computer. Trainz Plus (retail) which includes DLC and assets of my own creation, Trainz Plus (latest beta) which has a cut down local data folder and TRS22PE which has only built-in and DLS assets in its local data folder (no DLC or 3rd party). I develop routes in Trainz Plus (retail) and test them in the Train Plus (beta).
Before I upload anything to the DLS it is tested in TRS22PE to identify any dependencies that are not built-in or on the DLS. Any missing dependencies (i.e. Payware, Packaged in DLC only or 3rd party) are identified and then eliminated from the Trainz Plus copy and then tested again in TRS22. I have this dislike of finding dependencies in downloaded assets that cannot be found on the DLS which then forces you to go hunting for them or abandon the asset, so I will not inflict that on anyone else.