I have never lost at Ascension nor Shards of Infinity.
If a mouse walks onto the mortar (and fails to defeat Glorm), and his initiative comes up, does he use his regular attack? Or does he do nothing?
Like most M&M questions, you can always default to what would happen if the situation were real. In this case, if you were operating a cannon and had someone standing next you, you'd attack them hand-to-hand rather than use the ranged cannon.