First of all, try to relax.
Yes, mistakes can happen during production. It's easier than you might think to forget to install (replace) an ejector in the M&P. (Forgetting to install the ejector which slides back into the sear block, or the one that just rests against the left side of the sear block, depending on the caliber/model of the block.)
This definitely isn't the first time a gun's slipped through assembly & inspection with an issue, or simply a
missing or
wrong part, from one of the gun companies. Won't be the last.
Unless you
know how to remove the sear block assembly and install the ejector (and not just from watching some video clip), let the company do it under their lifetime warranty. They're pretty quick about it. (And it's less than a 5 minute repair, taking their time.)
It's not "completing" the pistol. It's "correcting" a production/assembly issue.
BTW, if you don't include the original magazines when you return your Shield for repair, they might include 1 or 2 of them when they return the gun. It's happened. Depends on whether they have any new ones at hand when the gun comes across the repair bench, though. Cross your fingers. (Other folks have found night sights installed on their pistol during a repair.)