Here's a video that covers it, though not in great detail (and a little dated): https://www.youtube.com/watch?v=SHVbx293eCs&list=PLzBixSjmbc... (this guy had a whole youtube channel on ADHD but it seems to have been hacked and deleted recently, which is a great shame. He's also talked about how in his experience a lot of the parents of kids he's treating also have ADHD, and how that can make things even worse if not addressed).
Yes. There's a reports from psychiatrists that specialise in ADHD that one of the problems that can arise is one or both of the parents having ( often undiagnosed) ADHD, something which can make parenting in general a struggle even if the child doesn't have it.
An here's a twin study which shows environmental effects are negligible compared to genetics: https://pmc.ncbi.nlm.nih.gov/articles/PMC4071160/ . I've seen a description from another reasearcher that it is "about as heritable as height" (and yes, environmental factors can affect that as well, but mainly it being reduced by malnutrition. With good nutrition, height is also mostly genetic).
Twins are always an interesting case, that could go against environmental conditions.
The first example doesn't hold much water for me personally, I wouldn't expect a reputable psychiatrist to make a scientific claim based on undiagnosed conditions...if they weren't diagnosed its not a viable datum.
For a random person I might not, but for someone who is specialised in ADHD and working closely with the parents I would rate it a lot higher, given those are the conditions for making a reliable diagnosis (and IIRC he then focuses on getting them formally diagnosed and treated as well).
