As far as chances an enemy spy in a city greatly reduces your chances of him getting in, asassination will greatly increase your chances.

As far as spies being killed, I guess it's just a game balance issue. If every spy you had that was found got killed, none would last too long. So for balance issiues they can only die of old age, failed mission or by the hands of an asassin.

The asassin is different and has a chance of living after a failed mission, then again how many asassins die of old age