I'm searching for a game that I can casually play during short 10-15 minute breaks from work on an offline Linux machine with mouse only.

This seems to be a good fit. The only thing I need to know: Can it be played without a keyboard?

Yes, you can easily play it using the mouse only (it's the way I play it most of the time and it works fine).

The games hotkeys are really helpful, but not strictly necessary. It can be fully played and enjoyed with the mouse alone.

Desktop Dungeons is far more fun when the player can utilize both mouse & hotkeys!

I play it comfortably on a touchscreen tablet PC, which has even less flexibility than just a mouse. In my opinion it plays great, and is a lot of fun.

I occasionally right-click to reset to default cursor mode. And you have to name your kingdom at the beginning. Afterward, I don't ever use the keyboard. I don't really like touchscreen because of precision needs, but mouse or stylus alone after your kingdom is set up is plenty 100% of the time. Keyboard shortcuts probably speed things up, but I never learned any and I'm fine with it. In fact, a mild UI rework was done for a phone and tablet version recently, and they definitely had non-keyboard interfacing in mind from the start.

I should have made myself clear ; try using a trackball mouse & keyboard to play Desktop Dungeons.

:)