Related Employer compensation

Our adoption agreement gives employers the option to exclude compensation from nonsignatory related employers.  I am trying to understand the practical implications of that selection.  From what I can find, I believe that regardless of whether this selection is chosen, there are some plan purposes for which you must combine all compensation from related employers:

415 testing

top heavy minimum contribution calcs

determining HCEs and Keys

calculating deduction limit

minimum gateway allocation

rate group testing

I believe that for ADP/ACP testing, the plan can choose to only use pay from the participating employer(s), as long as 414(s) testing is passed with that definition of pay,  EXCEPT there is a requirement for HCEs that all pay/deferrals must be combined for testing in every plan in which the HCE participates if the companies are related.

So it seems that the only practical implication of excluding pay from nonsignatory related employers is that it lets the employer calculate employer contributions on just pay from the participating employers (except for top heavy/gateway).  Is that correct?  I may be totally off base on this

While not totally off base, it is a complicated analysis. One thing I noticed is that you didn't limit gateway compensation to compensation earned while a participant. There are others. I don't have time to respond with a deep dive. Maybe somebody else can.