Hi, thanks for the reply. I'll try to answer your questions.
The reason one sketch "disappears" is because you are using History, i.e. timeline, i.e. parametric modeling. This is standard for parametric modeling. The model is built as a set of features that depends on everything before them, and when you "compute" the model it builds it in the order of the timeline.
So when that happens, you are editing a sketch that exists earlier in time than the one you see disappearing. You went back in time to do the edit.
The view of the timeline when editing an early sketch
(note, you can turn off the rotation effect in preferences)
The reason you might have more than one sketch is you may want to sketch on geometry that is created by using 3D tools on the initial sketch. If you edit the 3D feature in a way that changes the location or shape of the second sketch, you'd want the second sketch to respond by changing shape or location. That is history based parametric modeling.
All but the first sketch depend on 3D geometry
"Why not paste the object where the cursor is then activate Move with the distances from the original pre-filled? " This is not the case because a random click and location is typically useless for parametric design. Random is not useful, so why start off the workflow with random location information. For instance, if your original is at 0,0,0, and you paste a copy of it that needs to be at 15,0,0, but when you clicked it randomly filled out the values -3.5568459,2.65948984,0, it would be less convenient to remove the random information and replace it with the correct information.
What I'm going to suggest for you is Direct Modeling in Fusion 360. Go to preferences, and set your design history preference to "do not capture design history". I think you'll find a lot more familiarity when history is turned off. Please let me know what you think.
Turning off history in your new designs
Here is a link to more information.
http://help.autodesk.com/view/fusion360/ENU/?learn=foundational-concepts
Please let me know if you have more questions.
Regards,
Phil Eichmiller
Software Engineer
Quality Assurance
Autodesk, Inc.