If Mel Legend targets a mutated Mouser, sending the Mouser to -1 Might after the Mouser becomes a Defender, would he have 1 Might?

Yes, the Mutated Mouser would have 1 Might while it is a defender.

Here's why: Shield 2 is a passive ability (Rule 814.1) — it continuously applies "+2 Might" as an increase for as long as the unit has the Defender designation. It doesn't need to be "active" before Mel's effect.

Per the layering system (Rule 472.3.d), increases are applied first, then decreases. At the moment Mouser is a defender, the math is: 1 (base) + 2 (Shield increase) − 2 (Soul's Reflection decrease) = 1 Might.

Before it became a defender, Mouser had −1 Might (treated as 0 for game purposes per Rule 143.2.b). Once it gains the Defender designation, Shield kicks in and brings it to 1.