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I used MGN12H's so that the carriages don't stick out of the 3030 in case I want to attach an enclosure at some point. The MGN15H rail carriage would stick out. Yes, the MGN15H is solid and doesn't need any support in my opinion.
The Y flag can just be added on the top of the XY intersect and put in the holes that are already present. For the X end stops some sort of flag needs to be added to the XY Intersect. I am in the process of rotating the belt clamps by 180 so that the hot end faces the back which will change the position required for the X flags. I will post my changes soon.