Q: At what point, can non-certified firms perform electrical, plumbing, HVAC, or drywall work?

Yes.  Activities that do not disturb paint, such as applying paint to walls that have already been prepared, are not regulated by the RRP Rule if they are conducted after post-renovation...

Yes.  Activities that do not disturb paint, such as applying paint to walls that have already been prepared, are not regulated by the RRP Rule if they are conducted after post-renovation cleaning verification has been performed.