If there is no “certification”, how can I protect my company from liability?

From the OSHA Website

 Any employer who willfully or repeatedly violates the requirements of section 5 of this Act, any standard, rule, or order promulgated pursuant to section 6 of this Act, or regulations prescribed pursuant to this Act, may be assessed a civil penalty of not more than $70,000 for each violation, but not less than $5,000 for each willful violation.

Accessibility is a continuum, not a fixed point, and no document is as accessible to a sight-impaired individual as it is to a sighted one. If your will is to comply with the law, and you make a good-faith effort to comply with the law – contracting a reputable compliance service to remediate your documents to make them as accessible as is practical, using recognized best practices- you can not be made liable.