Where did you get the idea that the police in Seattle "aren’t allowed to arrest people for property crimes/trespassing etc."
So...you believe Neighbor A stole a package from Neighbor B? The first and most obvious thing that you "could possibly do" is mind your business. Another rather obvious thing that you "could possibly do" is report the alleged crime to the police.
Of course not.
That sort of depends on what sort of "attack" you mount. The law generally allows the use of reasonable, non-deadly force to prevent theft of property.
Sorry, but the notion that there is "no police protection" for this sort of thing is patently absurd.
Depends on all of the relevant facts and circumstances. Opining about incomplete hypotheticals is rarely productive.
Ah yes...Seattle...well known refuge for the lawless.

