Regarding your comment, its easy to build a domain model, but it's also easy to create a imperfect domain model, the following; That's one of the points where the collaboration between business and IT will be at the highest level. For me as a more business oriented modeler, that's the moment I seek collaboration with my techy colleagues. Since they can oversee the effect of choices.
Concerning your points about validation over multiple attributes and associations;
Those are validations on process level. You can model these validations within microflows. That is the more appropriated and fitting place, since those validations are depending on the process steps. Validations on domain level will always be checked, modeling them within microflows, gives you the opportunity to capture them within your process and make them depending on that process
Validation rules at the domain model, use them for validation of your rootdata
Well, in hindsight most of your concerns seem to have been addressed after all that time. I think it is justified to say your remarks have proven visionary.