1) It may have confused things if too big a deal has been made about her being executrix and such. The primary reason (in my opinion) that matters for this purpose is to establish her authority w/in the corporation to execute corporate actions.
Does the client have a copy of their certificate of incorporation? If not, for a small fee, you can get one from the appropriate agency in your state, likely the Secretary of State. Hopefully you are able to show that the incorporated entity is named in the plan.
I would politely point out that given the employer is the corporation which still exists, the plan is not abandoned, cross-reference 29 CFR 2578.1 http://www.law.cornell.edu/cfr/text/29/2578.1 and then challenge the custodian to substantiate their position.
2) You don't indicate what level of person you are dealing with at the custodian. If in doubt, escalate. If necessary, insist on speaking to their legal department. My position would be: if they need a different corporate resolution to be signed, you'll be happy to get it, but otherwise you need the citation for the Code or Regulation that supports their refusal. (And I would use the word "refusal", they are deliberately refusing your valid document.)