Is it allowable to amend a cross tested plan to allow owners (through family attribution rules) to opt out of participating in the plan, so as to allow higher allocation rates for those remaining? If so, what are the operational requirements (signed opt out forms, etc.)? Any cites also appreciated.

Purely a function of your plan document. Some allow participation waivers, some don't. Timing and operational requirements should also be specified in the document language.

Can you add a group to your doc group breakdown and group them together by definition. Then give that group $0 or 0% for an allocation?

Or is it a mix and match where not all in that category want to be included/excluded?