If I cast a single target instant/sorcery spell on a Mirrorwing Dragon and I have Silverfur Partisan in play, do the Mirrorwing spell copies trigger Partisan's ability? (assuming that Silverfur Partisan is a legal target for the original spell, of course)

I think changing targets is the only thing that's supposed to not trigger "becomes the target" abilities. I could be wrong, though.

Even that isn't an exception, provided the creature wasn't already the target of that spell/ability.

114.6. Some effects allow a player to change the target(s) of a spell or ability, and other effects allow a player to choose new targets for a spell or ability.

114.6a If an effect allows a player to “change the target(s)” of a spell or ability, each target can be changed only to another legal target. If a target can’t be changed to another legal target, the original target is unchanged, even if the original target is itself illegal by then. If all the targets aren’t changed to other legal targets, none of them are changed.

114.6b If an effect allows a player to “change a target” of a spell or ability, the process described in rule 114.6a is followed, except that only one of those targets may be changed (rather than all of them or none of them).

114.6c If an effect allows a player to “change any targets” of a spell or ability, the process described in rule 114.6a is followed, except that any number of those targets may be changed (rather than all of them or none of them).

114.6d If an effect allows a player to “choose new targets” for a spell or ability, the player may leave any number of the targets unchanged, even if those targets would be illegal. If the player chooses to change some or all of the targets, the new targets must be legal and must not cause any unchanged targets to become illegal.

114.6e When changing targets or choosing new targets for a spell or ability, only the final set of targets is evaluated to determine whether the change is legal.

11.1. A spell is a card on the stack. As the first step of being cast (see rule 601, “Casting Spells”), the card becomes a spell and is moved to the top of the stack from the zone it was in, which is usually its owner’s hand. (See rule 405, “Stack.”) A spell remains on the stack as a spell until it resolves (see rule 608, “Resolving Spells and Abilities”), is countered (see rule 701.5), or otherwise leaves the stack. For more information, see section 6, “Spells, Abilities, and Effects.”

111.1a A copy of a spell is also a spell, even if it has no card associated with it. See rule 706.10

706.10. To copy a spell, activated ability, or triggered ability means to put a copy of it onto the stack; a copy of a spell isn’t cast and a copy of an activated ability isn’t activated. A copy of a spell or ability copies both the characteristics of the spell or ability and all decisions made for it, including modes, targets, the value of X, and additional or alternative costs. (See rule 601, “Casting Spells.”) Choices that are normally made on resolution are not copied. If an effect of the copy refers to objects used to pay its costs, it uses the objects used to pay the costs of the original spell or ability. A copy of a spell is owned by the player under whose control it was put on the stack. A copy of a spell or ability is controlled by the player under whose control it was put on the stack. A copy of a spell is itself a spell, even though it has no spell card associated with it. A copy of an ability is itself an ability.