Hi there, I'll add some more text here when I get time. 

You are right, for new users, there are too many ways to work in this area and have work planned for vm to make this process easier to manage and use.

Initially you could try ignoring the solution side of things. never both saving the solution when prompted or always save to the suggested Solution1 in the prompted vs my documents area.

In that mode you would always use File>Open Arduino Sketch" or "File>Open>File>.ino" to re-open a sketch. Once a sketch has been opened once you can continue to open the sketch that way or you can use the std. The File>Open Arduino sketch opens the Sketch Book Folder for you to select a sketch but it does not add the sketch to your recent files list

After a sketch has been opened once VM will create a VS project in the sketch folder. Using the std. vs File>Open>Project will open the project and add the project to your recent files list. 

Vm doesn't care how files are opened in VS, the additional menu items are just there to help some people, other will use the std. vs File Menu, Context Menu and Tool bar functions. 

All the std. VS (non-arduino) functions "add to the recent files/projects list" on the File Menu. This can often be the most useful workflow for opening any of the 10 or 20 most recent Arduino Sketches

To be clear: The sketch.ino and/or project (.vcxproj) are the important items when it comes to opening files. In theory you can ignore the solution side of things entirely and find the way to open your sketches that works best for you.