The plan document (Sungard) requires participating employers to give written approval for all amendments. This has not been done. Only the primary employer and trustee have been executing amendments. The plan has terminated and will be submitted for a letter of determination. How big of an issue will this be? Should some type of corrective action be taken in advance of the submission?

I don't think the IRS will comment at all. I would do nothing. But, if you are concerned, I would, by Board resolution, have the participating employer confirm that it accepts all amendments made by the sponsoring emplyer and consents to those amendments.