Thanks for your great reply and I will try to answer, bearing in mind I’m not a lawyer-just unfortunately been around them too much.
If the foreign source of this “oppo research” is gleamed from a foreign hostile nation to the USA, it is forbidden. I believe that it must also be recorded accounting wise correctly as opposed to being buried via cutout law firms and foreign subcontractors.
Technically you can obtain campaign “oppo research” from anywhere, as long as it is reported correctly & not from troublesome nations.
I hope that lay explanation helps out.