Re: mark as read
Do you have Gmail keyboard shortcuts enabled?
If so, after you've selected all the messages/conversations you wish to mark as read, you can then hit SHIFT+i to mark them all as read.
Brian - Windows 10 Home, 64-Bit, Version 1809, Build 17763
Explanations exist; they have existed for all time; there is always a well-known solution to every human problem — neat, plausible, and wrong.
~ H.L. Mencken, AKA The Sage of Baltimore