Dear Dark,

My answer to your query.

1.Yes, arrangement of the spare piping does affect the nozzle of the spare pump.
2.I will not recommend to use the EJ.

The spare pump could be any of the three pumps.

I had analyzed 3 pump system.It is always difficult to limit the nozzle loads within the allowable limits.
You should reroute the piping in such a way that pump piping of all the three pumps is symterical with respect to the piping header.Provide a line stop in the middle of the piping header & check if it helps.

Did you consider extra allowables for the spare pump?.
I believe you can consider extra allowable loads for the spare pump.In my system as per the project guidelines,I had considered 50 % extra nozzle loads for the spare pump as compared to the working pump.

Hope it will help.

Cheers
Saneh