As is, ENTER will get all selected images into the viewer.
The left and right arrow keys on the keyboard facilitate moving amongst the images.
Zooming (10%-800%) is done by moving the mouse left or right while the ALT key is depressed.
Is there any way that the zooming can be brought about by the up and down arrow keys (instead of moving the mouse to the left or right) while the ALT key is depressed? (ie., program a keyboard shortcut for the up and down arrow keys that mirrors the left and right mouse movements, for the case in which the ALT key is depressed)
I suggest you use the <Ctrl>+<+> and <Ctrl>+<-> or one of the many other keyboard-based zoom methods. You can also use the context menu via right-click > Zoom.
See the Viewer help topic or the Keyboard Cheat Sheets (https://www.photools.com/help/imatch/cheat_sheets.htm) for a list of all available keyboard shortcuts.
The free <Alt>+Mouse zoom mode is only available for mouse.