Question: Is the mouse supposed to be the re-incarnation of the dead man that left the mansion to the brothers?
Question: The mouse was deliberately put in a parcel without airholes. How did it survive?
Answer: It survived because it's funny. Just like it survived everything else the brothers tried to do to it.
Question: What year does this movie take place?
Answer: There's not really an answer, because the film has sort-of a nebulous timeframe given its cartoon-like atmosphere. Ex. Everything is old fashioned in design, including architecture and clothing, but current technology is also present. It's not really meant to take place in the "real world" or a specific timeframe, so much as be a silly throwback to old comedies and especially old Laurel and Hardy films. So, the best answer to your question would be that it just sort of exists in its "own" time, so to speak - an amalgam of different time periods all put together for maximum comedy impact.
"Mouse Hunt" was not modeled on 1930s Laurel and Hardy films so much as it was a direct retread of 1940s "Herman and Catnip" and "Tom and Jerry" cartoons, which spawned many knock-offs over the decades, from "Coyote and Roadrunner" cartoons to live-action comedies such as "Home Alone" and others.
Answer: No, it's just an intelligent and precocious mouse.
LorgSkyegon