I’m really busy (same as most people), and get an average of 50 emails every morning in two email accounts. Most I’d like to simply glance at and MARK AS READ. Almost every day, in a rush, I click on the adjacent FLAG option by mistake … and have UNFLAG then MARK AS READ the boilerplate stuff I gave. I’m in rush, and that won’t change. How about you move the FLAG option in the menu some distance from the MARK AS READ I have to use in great speed and volume every day? Thanks in advance - 
Hi .
This is easy to do.- Dependent on how you work.
This doesn’t change the Context menu, so instead use the Toolbar menu.
I’m sorry - I don’t know how to edit the Context menu.
*As an alternative - Read the additon at the bottom of this post.
R-click an item on the main toolbar - and click customize
You can do this for the Headers of any folder as well
R-click any column header > Customize
This was done in Inbox - also showing main toolbar
Result 
Added: I reread your post an by adopting a different method you may achieve what you want.
Reduce the time that the message automatically turns Read.
i.ow, Forget about the R-click or Context menu method…
This is faster than what you do now.
Menu > Tools > Settings
I think 1 or 2 seconds may work. ( “Most I’d like to simply glance at and MARK AS READ” )
If you think it might need followup - Flag it The flag symbol appers faint in the message line - click it and it’s flagged. as per above graphics
Peter