If I cast Mortars for its overload cost and copy it with the Guildmage's first ability, will it be considered an "overloaded" copy and have "target" replaced with "each"?
Quote from Magic: the Gathering Comprehensive Rules (2012-10-01) »
702.94c Overload's second ability creates a text-changing effect. See rule 612, "Text-Changing Effects."
Quote from Magic: the Gathering Comprehensive Rules (2012-10-01) »
706.2. When copying an object, the copy acquires the copiable values of the original object's characteristics and, for an object on the stack, choices made when casting or activating it (mode, targets, the value of X, whether it was kicked, how it will affect multiple targets, and so on). The "copiable values" are the values derived from the text printed on the object (that text being name, mana cost, color indicator, card type, subtype, supertype, expansion symbol, rules text, power, toughness, and/or loyalty), as modified by other copy effects, by "as . . . enters the battlefield" and "as . . . is turned face up" abilities that set characteristics, and by abilities that caused the object to be face down. Other effects (including type-changing and text-changing effects), status, and counters are not copied.
Example: Chimeric Staff is an artifact that reads "{X}: Chimeric Staff becomes an X/X artifact creature until end of turn." Clone is a creature that reads, "You may have Clone enter the battlefield as a copy of any creature on the battlefield." After a Staff has become a 5/5 artifact creature, a Clone enters the battlefield as a copy of it. The Clone is an artifact, not a 5/5 artifact creature. (The copy has the Staff's ability, however, and will become a creature if that ability is activated.) Example: Clone enters the battlefield as a copy of a face-down Grinning Demon (a creature with morph {2}{B}{B}). The Clone is a colorless 2/2 creature with no name, no types, no abilities, and no mana cost. It will still be face up. Its controller can't pay {2}{B}{B} to turn it face up.
As you can see, Overload creates a "text-changing effect", which is not a copiable characteristic. Since the copy was not cast for its Overload cost, it behaves as any other non-overloaded spell would.
If I cast Mortars for its overload cost and copy it with the Guildmage's first ability, will it be considered an "overloaded" copy and have "target" replaced with "each"?
As you can see, Overload creates a "text-changing effect", which is not a copiable characteristic. Since the copy was not cast for its Overload cost, it behaves as any other non-overloaded spell would.