I would use the “load optimized” button for the internal camera parameters only if the camera is not in our camera database yet and if you would like to create a new camera model for it. This involves adding Manual Tie Points (MTPs) as references and do some iterations until the optimization does not change anymore. The MTPs are like a reference that can be used in the optimization, that’s why I would suggest adding them before optimizing the parameters. The detailed procedure is described in this article: https://support.pix4d.com/hc/en-us/articles/206065716
If the camera is in the camera database and there is a large difference between initial and optimized parameters, it is tempting to “load optimized” parameters to reduce the percentage displayed in the quality report…the problem with that is that you were able to remove the flag in the quality report, but the software has ended its optimisation at the same camera parameters. The percentage displayed is just smaller because the optimized parameters from the previous trial was already close to that “optimum”, i.e. nothing changed from when there was a large difference in the camera parameters.
If the camera model from the Pix4D camera database is used, I would take a different approach to reduce the difference between initial and optimized parameters. In some cases, the optimization of parameters can have more than one optimal result: one close to the initial parameters and one further away. This often happens with DJI cameras. In that case, you can use the “All Prior” option in Process > Processing Options > 1. Initial Processing > Calibration > Internal Parameters Optimization.
There can also be other reasons for large differences in the camera parameters, depends a bit on the case, but the above hints should help in a majority of them.
As for the order in which Ground Control Points (GCPs) should be added. I generally add them after step 1. Initial Processing, because it’s easier to mark them. Then, just make sure to click Process > Reoptimize. Otherwise they are not taken into account. Note: you can also add them before step 1 in the Basic Editor, but I’d only use it if there is no other option.
Hope it helps!