The server performs the needed hashes to determine which bits of the nonce are the correct bits to be sent back to the user and keeps the sneaky bits.
...
its still quick enough for a share or so per minute of normal mining, OR it could be used for a speed boost for your own setup finding a valid nonce through other means by eliminating work exponentially, then the next encrypted work abort that sneak, etc etc.
No.
This is the
double compression fallacy. The only way this works is by my servers doing part of the hashing work, in which case
what the heck is the point?