Yes, as a Delta codeshare partner you would list as S3D and that will allow the gate agents to clear you before all S4's including Delta employee buddy passriders...and it entitles you to coach class only...if you are on a thru flight with same flight number you would be above all S3D's and S4's who would board at that stopping point, but you could be pulled off for anyone traveling on higher priority...
The issue with coach vs 1st or business is this - it all depends on the gate agent working the flight...I know some gate agents who were "wronged" (their words not mine) and placed in coach on skyteam or codeshare flights with 1st or biz wide open, and will only put those employees in coach, then there are agents like me who will put anyone in 1st or biz they can because it is the best thing to do...the issue is the S3D is inhibited and will not allow you to be placed in 1st or biz, the gate agent has to change your priority to S4 to be able to place you in 1st or biz...