@1. -Install AdvancedEditing plugin;
-Select the polyline;
-Edit points...
-New (on bottom left).

@2. To have the length I use HORIZONTAL cylinder, export to CSV then in Calc or Excel make the sum;
I use this also for pipes.
NOTE: the cylinder MUST be horizontal in library to keep the correct length in any position or angle.

@3. Read @1 & @2.
Good luck and patience!