How large is the gap?  Anything within 0-4 percent is well within industry standards.You will always have a gap due to users on ad blockers or using browsers like Brave.  A gap can also be caused by internal users purchasing (if you have say an internal IP filter).I would initially focus more on set...

It appears you are in the incorrect area for what you are tying to acheive.  The channel logic would be defined in the 'Channel Processing' area.The classification area is more frequently used to set the components of a channel.

Lets say you had a list of pages in your rows.  And you wanted to always viz values for the home page.  If you chose 'selected items' it would always render a viz based on that value of the dimension.  If you selected position in that circumstance it would always return the value in that position (s...

C.  Segment stacking.

Rather than selecting the metric, select the sparkline immediately below it.  After taking that action the option to lock selection should return.Similar to most platforms you may select non consecutive items by using the ctrl key.