If a PERM application was recently certified but the employer modified the job to a fixed-term/temporary role, can a private attorney still file the I-140/I-485?
No. Employment-based Green Cards are strictly for prospective, "permanent" future employment. Because the employer does not intend to continue employment permanently, an I-140 cannot be legally filed based on that PERM. You must abandon this Green Card process and urgently find a new H-1B sponsor.