Hi Doublezer0,
Unfortunately what you wish to do isn’t really supported by the engine. In general, the engine adheres to the philosophy that the User Construction Script is not a set up step that can then be customized on a per-instance basis. To support that would cause confusion and fighting over the ownership of properties (the instance overrides vs the archetype blueprint). As you can see, any component property changed by a Blueprint’s Construction Script becomes read-only, and splines were brought into line with this in 4.9.
Perhaps what is needed is some kind of utility which relinquishes control of an actor from the blueprint to the instance, although this of course means that any changes made to the original implementation (the archetype) wouldn’t apply any longer. If Bluefabs find their way back on to the roadmap soon, this may be the solution you need.
Sorry not to be able to bring you better news. This is an area which is constantly evolving to meet users’ needs, and I will certainly put forward your use-case as an example of why we may need to modify our model a little.