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I don't really know to answer your first question, but I would suggest to try running several SLAM algorithms and keep as ground truth the one that you think performs better.

As for the second question, there exists this package https://github.com/robotics-4-all/ogm_merging that implements the evaluation between two different OGMs.
It is applying the Nearest Neighbor method, for every point of the map that is considered to be an obstacle, calculates the distance between these points and finds the Overall Mean Square Error.