1. Microwaving it would be relatively humane if it dies in 5 seconds (brain overheat most likely). There's plenty of worse things to do
(A 1kw microwave for 300 grams of rat, when scaled with mass is - 0.25 Megawatt for an 80kg human. - The maximum power output of a
https://en.wikipedia.org/wiki/List_of_tidal_power_stations small tidal power station. It's a pretty humane and fast way to die.)
a. just throwing it with with the trap and letting it die of starvation
b. incinerating it with a lower powered heat source (slower and heats from the outside in) etc.
c. Use underpowered fireworks
d. Any >5 sec death
2. Should you be focusing on minimizing pain?
3. If the rat could, it would happily eat you alive without giving a single shit.