Honestly, these people need some divorce planning. If the house is being sold, that could be done before the divorce, so they get the marital exemption and now everyone starts at the same level.

"To determine house ownership, do I get and read the trust document?"

I don't see where you previously mentioned a trust

"Do I ask for the deed to the house?"

Yep. Start with the Deed (title). If the property is deeded to the Trust = done. Now you need to know what type of trust, and what is in the trust. And that might mean reading their wills, in case they did a "pour over" of their Estate into the trust. Is the trust already filing tax returns?

"An ADU is being built - does that have a separate deed?"

You'd have to check with that County.

"What is the legal document needed to verify C-Corp ownership?"

Shareholders. Corp records, K-1 from the C Corp on their personal tax return, etc. For instance, you would find out if they hold Ford, or other investments, as publicly traded investments, and "closely held" investments, and what other assets are involved for purposes of the divorce. If you've been doing their personal taxes, you should already have a handle on most of this.

You're not handling the divorce, though. Are you sure you want to go down this rabbit hole?