Hi @Stan and welcome, we are so glad to have you on the HubSpot Community!
And excellent questions, thanks for asking! We also appreciate your research before posting @Stan! I’d like to thank as well @karstenkoehler, @profitpad and @Emedion for their valuable contributions, so helpful! 
I can confirm what has been shared here, you can do a post-stage automation on associated records, but at the moment, there isn’t a pre-stage enforcement based on associated Contact or association data.
I’d suggest commenting and adding your upvote on the idea that @karstenkoehler shared here: Make properties on associated records required when moving a deal to a deal stage
Currently, in a deal-based workflow you can absolutely enroll/filter using associated-record properties and association labels, and you can copy or update values across associated records.
Using playbooks, as you mentioned @Stan, can also help with the data-entry step, including updating properties on contact/company/deal/ticket records, and you could let the rep choose which associated record to update when logging the playbook.
But this doesn’t make the deal stage transition itself wait on associated Contact completion. Conditional property logic is enforced when users manually edit records, including in playbooks, but it applies to the record being edited and does not apply when records are edited by workflows.
Likewise, the sync properties can surface associated Contact data onto the deal as read-only values, which is useful for visibility and downstream rules, but they don’t solve the “block the stage move until associated Contact data is filled in” problem by themselves.
So, you could store stakeholder roles on associated Contacts or labels and use sync fields or playbooks for guidance, but true enforcement (blocking a Deal stage change until Contact data is filled) would require a custom UI extension or app.
Let’s consult additional Top Experts: Hi @danmoyle and @Anton do you have any real-world examples or architecture suggestions or other workarounds to help @Stan, please?
Thanks so much and have a lovely day!
Bérangère