When receiving 417 it should be re-trying the POST without the expect header, or the delays Expect adds to the system.
a- What means "or the delays Expect adds to the system" ? To wait answer to Expect request ?!
Squid other than 2.7 will not produce this error. They will just die in unexpected ways if the 100-continue your software asked for actually comes back.
b- You are saying Squid process (a version different from 2.7) will die if it receives a request which contains "Exepct: 100-continue" ? c- But the 417 error is given from Squid or from remote Http server ? I suppose, in this case, the error is given from Squid 2.7
