I'm under the impression that if you get the 5500-EZ out by December 31, 2019, they'll let you use the 2018 form, but if the calendar flips to 2020 then you have to wait for the IRS to release the 2019 form.
The controlled group analysis crosses international boundaries. The stuff in 1563 about foreign affiliates not being "component members" being disregarded is irrelevant to the Section 414(b) and (c) issues. Of course, nonresident aliens with no U.S. source income can be disregarded for 410(b).
A plan that does not cover employees is not a plan subject to ERISA, but of course that is putting the rabbit in the hat and there is no need here for a dissertation on the potential adverse consequences of worker misclassification.
To me you had it in your original comment. It says comp paid while not a participant. Your description of the facts says it was paid while a participant. It doesn't say earned but paid.
What is the plan's definition of compensation in the document? If compensation is on a W-2 basis, it should be easy to explain since compensation for W-2 purposes is based on when PAID, not when earned.
So partial year compensation would similarly be based on the period in which it was paid, not when the hours were actually worked.