ownership Ownership for a company held by many holding firms?

We requested ownership information for a client that onboarded with us this year. Most of our plans are fairly simple with ownership as we specialize in small plans (I don't think we have a single plan that's large enough for a large plan audit), so it's nearly always just some split between a few employees. 

This company sent over a cap table, showing columns for common & preferred stock ownership, outstanding and diluted shares, etc for ~60 lines, some of them being employees holding stock, but also a large number of them (40+ or so) being holding firms or other entities that aren't individuals. A few questions.

1. When calculating ownership for 401(k) purposes, is ownership specifically voting stock? Their preferred shares don't have voting rights, so that would impact how ownership % is calculated (CS / total CS vs common & preferred / all stock). 
2. How should we handle the potential of control groups? While unlikely, if a combination of those holding firms all held interest in another company that added up to 80%, couldn't we run into a control group issue that we'd have no way to know about?
   1. In this case, I believe the 80% ownership for controlled groups is specifically between 5 or fewer individuals/entities. If the 5 largest stakeholders don't add up to more than 80%, would it be safe to assume we're safe in this regard, as no combination of 5 firms could hit 80% anyways?
3. If someone who held stock at this company also was an owner of any of the holding firms, would they then need attributed ownership from that? E.g if John Doe owned 5% of the shares of this company, but also held 50% of ABC Holdings, which held 10% of this company, would John's ownership be 5% (direct) + 50% * 10% = a total of 10% ownership for plan purposes?

While the odds of any of this being particularly relevant is fairly low, especially in the small plan world, I'd prefer to have a solid understanding of their plan ownership; any input would be appreciated. Thanks!

The topic you ask about is full enough that Derrin Watson wrote a whole treatise, and over 28 years has revised it. Who’s the Employer https://www.erisapedia.com/static/WTE.pdf.

While it’s tempting to seek a shortcut, a plan’s sponsor, participating employer, or administrator wouldn’t know which organizations and businesses are in or out of “the employer” until checking everything.

Consider limiting your scope to what affects the design and administration of the one retirement plan you work on.

Consider warnings that your work must not be relied on about how discerning who’s-the-employer for the one plan you work on affects employee-benefits plans of other organizations and businesses an owner of your client owns, whether indirectly or even directly.

This is not advice to anyone.

If you need advice, consider Ferenczy Benefits Law Center.