I don't see how that applies in this context which is pretty simple. The transaction with Don Jr. was a board advisory role in exchange for equity. Broadly, bribery is "paying to influence the actions or decisions of a public official." Don Jr. isn't an official. He signed a contract to provide board advisory services. The contract wouldn't say anything about anything else. I've been a board advisor and signed a similar contract (if not the same one, they're quite pro forma).

Absent a secret recording establishing payment for influence, there's nothing to prove bribery. Do I think making him an advisor was done in the hope it would influence his father to promote and sign favorable legislation? Yep. Do I think there's any chance evidence exists to prosecute that? Nope.

A Democrat controlled congressional committee can certainly issue subpoenas, have a public hearing and ask Don Jr and/or the company CEO a bunch of leading questions - and all the answers will deny any quid pro quo or wrongdoing. Personally, I think the companies probably knew giving Don Jr. a bunch of money wasn't a very reliable way to ensure Trump's cooperation. It's quite clear that Trump demands loyalty from all but gives none in return. I think their main bet was that hiring Don Jr. would cause potential investors and business partners to infer the company had lower regulatory risk, making the net cost of investment capital lower. Dumb, but that stuff actually works to some extent.