I think you have the best practical advice. If approached by anyone not a citizen of the United States, who says they have dirt, it’s best to go through intermediaries. For example, had I been Don Jr. in this situation, here’s how I would have played it:
You get the e-mail from the British intermediary. You thank them politely for the offer. You tell them that you will consider the offer, and that they can expect to hear from someone regarding the offer. You have a fairly low level staff person contact a staff person of a “friendly” news source, say Sean Hannity. Tell them about the source, and have Hannity’s people contact them.
If they want to play ball that way, then let it happen. If they don’t want to, then the whole deal was either a set up or a shakedown. You and your candidate do not get any stink on you under those circumstances.
To me, it looks like D.T., Jr. was set up by the opposition, with the approach to him crafted by Clinton operatives with the benefit of access to US intel material on the Trump family and campaign. I suspect that the key point of the maneuver was to implicate him as eager to get help from a foreign government.