Since Rob is a foreign national (in the UK), this might be facilitated via the Hague Service Convention or Hague Evidence Convention.
https://travel.state.gov/content/travel/en/legal-considerations/judicial/country/united-kingdom.html
Speaking of receiving things of value, public officials are prohibited from receiving such from a foreign state or third party - so they can’t be influenced in making decisions on behalf of the country.
Rule doesn’t apply to private citizens. Both Don Jr. and his dad where private citizens in June 2016.
No law was broken.
Agreed, that seems to be the only thing that really matters.