My editorial was taking a very high level view, and says that in the end, strictly speaking, noone can validate LCA results in the sense that you see "in reality" how the environmental impacts over the life cycle of a specific product are (and compare this with the calculated / modeled result. That said, there are practices to check if datasets or studies are correct, compliant with ISO for example, and these work. I would not make the intitial evaluation too technical (so not start with a specific GSD) but it is often first important to see whether flows are as you expect etc., or, for studies, impact assessment results. Also here, for datasets, I can refer to a an article: Ciroth, A., Foster, C., Hildenbrand, J. et al. Life cycle inventory dataset review criteria—a new proposal. Int J Life Cycle Assess 25, 483–494 (2020). https://doi.org/10.1007/s11367-019-01712-9
To your last point: A five time five quality assessment is of course not good, but this assessment reflects the ecoinvent default assessment (fitting to the goal and scope of ecoinvent) which can be different from goal and scope of your study. Not for reliability of the source, typically, but for location and time, possibly.