The "no process killed" error is normal. Basically John has a nasty habit of running even after it has closed, so I implemented a basic workaround of killing the process each time you click the decrypt button. I am implementing a far more elegant solution in V0.4. I am also giving you the ability to specify the number of digits for John with the new version.

There is no limit to the amount of digits you can use if you just load a dictionary instead.

That's why you use John. It creates the dictionary and pipes it to the stdin of aircrack in real time.

You are correct though that it is not likely to work (in a reasonable amount of time) on a device like the N900. The only chance you have really is if it is just numbers and if you know the exact length of the key. From personal experience, most default passwords are either 8 or 10 digits long, although I have seen routers with passkeys of 32 and even 64 digits by default, although these tend to be only on industrial/business models. Most routers are made to be as simple to set up as possible, and Average Joe is not going to be happy having to type in a 64 digit key just to set up their connection for the first time.

Incidentally, I created a dictionary containing every number from 00000000 to 99999999 and it comes to 854.3MB, which is not too bad.