I doubt pouring bleach will solve the problem: unlikely to actually get to the decaying corpse. If there is a dead body in the walls, then you either have to open the walls to get it out, or wait until the decay process is done. It might (might, maybe) be helpful to close up the holes that are letting the stench out.
Are you sure it is just a mouse and not something larger? Mice do smell but generally not for a long time. Not a lot of flesh to rot, so it tends to go quickly.
Could be worse: my cats dragged in a rabbit once, and it died under the fridge. Took me a lot of searching to find the source of that smell, and it was a powerful one.
You do have my sympathy.