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Thanks for getting this model, I hope you enjoy building and flying it.;)
About your issue. I haven´t got this "non-manifold" edge with Cura, Everything was ok. I will check it with Simplify and I will let you know, although I see that you have already solved this problem.
About the bottom tab of FUS5 (with WingC you will probably have the same problem) I recommend you to increase a bit the flow, this way the wall thickness will be greater and will ensure the fixation. Using Cura with 100% of flow you already get more than 0,4 mm of wall thickness (you get around 0,48) and you won´t get that problem.