How do I analyze "是其于主也至忠矣"?
This is how I read this passage:
公行之計是其於主也至忠矣

[If] you/公 use (行) this (之; read it as 此) strategem (計), this (是) inevitably [shows] (其; read it as 乃 or 然) to (於) the lord (主) pause (也) great loyalty (至忠) ! (矣)
Using this strategem is a display of [your] loyalty/devotion towards the lord.