> He is approx 5cm long
Help me. 5cm is 50mm which is two of 25.4mm which makes two inches scant?
That's a mouse. Maybe better-fed than some, but not too large to be a mouse.
No wonder he can gnaw rat-traps. Not big/strong enough to trip them.
And yes you MUST core the food with something inedible and tough. For mice, a scrap of paper-towel rolled to a half-toothpick and soaked in nut/fat/salt. Rats may need string, like you tie-up your newspapers.
If it were/is a rat..... Isn't there an old-spouse's tale, that rats can't burp? That if allowed to drink their fill of beer, they die of gas pressure? Against that theory: if beer killed rats, the Black Death couldn't happen. Everybody drank beer and bottle-caps had not been invented. (Or was medieval beer served flat?) Also the MythBusters filled a pig stomach with Pepsi and Mentos, did not get explosion.
There is a plastic tip-over trap for mice. They only have to get halfway to the bait, then it tips, door drops. Yes, they have a Last Meal then starve to death, dump their bowels, rot. You have to check often, and be prepared to deal with the trapped beast dead or still-screaming.
For yet another option (with good and bad points), RatZapper.com has been on the web a long time (since NetScape 0.9 at least) and is still in business.
> shotgun
I'm good with a long-gun against paper targets, but the several times I have stalked mice they moved MUCH too fast to swing a long-gun. A 2-foot spray is no good if it is 4 feet behind the tail (though the noise may spook them). Maybe a very cut-off shotgun, but that's usually associated with Criminal Intent, anyway still a lot of mass to swing.
The "Lightweight Spackle" is much easier to use for filling such damage. Won't fall out of the hole. Doesn't shrink. Won't take a fine surface like drywall mud, but if this is how you control rodents (or you have wild tenants) maybe that's less important.
> Rat poison is generally blood thinners like Warfarin
Most everything has been tried on mice/rats. Several of the "poisons", in smaller doses, have many many effects, some interesting. I know some (more than I want) about sub-fatal strychnine. It isn't a lot of fun, but maybe more than the "bath salts" which are in the news.
The other thing about poisons is that dogs/cats get it, either direct from the bait-station or by nibbling the "easy prey" staggering around or laying there like food.