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I think this is OK. The key is not to have any pre-tax IRA to avoid the pro-rata. The devil is in details as usual.

I don't see any reason why you couldn't amend the definition of compensation mid-year for employees who have not yet entered the plan. However, the plan's definition of compensation for allocations does not (usually) control its definition of compensation for testing. You can use post-entry comp for testing without amending the plan. You can also use comp net of deferrals (remember to also net out sec. 125 deferrals if you do this). Just double check that there isn't something in your plan document that would lock you in to a particular definition of comp for testing. If you think about it, though, allocation comp for deferrals is always post-entry comp; you can't defer on comp that was paid before you entered the plan.

Even to explain one plan, the Employee Retirement Income Security Act of 1974 permits a plan’s administrator to use more than one summary plan description, making them distinct by clearly identified classes. Each SPD may omit information that cannot apply to the class of participants or beneficiaries to which that SPD is addressed. 29 C.F.R. § 2520.102-4 https://www.ecfr.gov/current/title-29/subtitle-B/chapter-XXV/subchapter-C/part-2520/subpart-B/section-2520.102-4. But your focus on which benefit a participant enrolled in seems inapt. Rather, an SPD must describe each benefit its addressee could elect.

Are you sure that's what the plan says? Read the exact wording in your plan document. I bet it actually says something to the effect that non-resident aliens with no U.S.-source income are excluded. If someone worked in the U.S. then they would not fall under that excludable employee classification, even if they are not a citizen and not a permanent resident.