Use integrity checks to ensure that the slaves are acting properly. One method of doing this is to keep secret part of the known plaintext (say 16 bits). A slave is required to report _all_ matches in the range to the master. Slaves who report a statistically low number of matches may be considered suspicious. It is a simple matter to allocate part of that keyspace to another processor for a double-check.
Please don't do anything like this. This will prevent people like me who prefer the "random" method from participating.
Not true, it would be open for anybody to sweep a random space and report the results. The only difference would be that the sweeper who discovered the real key would not be the first to know of a break and that it would not be possible to attack the crack through dishonestly claiming to have swept space that hadn't been. Phill