On your two questions:
1. The paraphrase and quotation are from the rule cited, 29 C.F.R. § 2550.408b-2, which interprets ERISA § 408(b)(2). The hyperlink points to the government’s Electronic Code of Federal Regulations rendering of that rule.
2. The focus is on whether the fiduciary that engages a service provider makes a fully independent decision. Even if neither the service provider nor its owner-operator is the engaging plan’s fiduciary, one might consider whether a desire to please the owner-operator, perhaps because she is company B’s minority shareholder, could tempt a decision-maker to use less than her best judgment for the retirement plan’s exclusive purpose.
These are sensitive questions, and each party to the would-be service agreement should want its own lawyer’s advice.