You're missing these standard menu commands and shortcuts for multi-selection...

⌘A - Select all (in a given view, like inbox)

⌘⌥A - Deselect all

And you have ⌘-click for discontiguous selection, but are missing ⇧-click for contiguous (for example, select the first in the view, scroll down, ⇧-click on another to select everything in-between inclusive.)

Oh, and ⇧-Up/Down/Left/Right should extend the selection in the direction of the arrow pressed.

FWIW You can try all these behaviors in Finder, which uses them in a mostly standard way and many Mac users are familiar with.