The XYZ model that is created by Roomplan is perfect for CAD. We have found the accuracy to be within 3% (apple quote 10%) Yes items are tagged

The apple RoomPlan can not be extended but you can add your own library and make it operate alongside RoomPlan, I am currently adding Yolo3 objects to the recognition process, to create an extended recognition, including boxes.

Yes see https://developer.apple.com/forums/thread/708591

I seems to often work to 15cm, it has a claimed accuracy of 90%.

Yes 3D drawings are X, Y, Z to give a plan in 3D. Its not normally as simple as changing Z to 0, but there is formula that will do the translation, what is your need?

Could be interested.