Why did Michael Waltz, a former advisor to Dick Cheney, seek to add war-pimp and anti-Trumper Jeffrey Goldberg to his contact list? What did he plan to leak to him?

I believe this was a veiled message to Europe. Trump knows if he used the press channel, his message would be warped and misinterpreted. This way, he could let the world know, that his people are backing him up and the US is resolved. This was also a way to say things, you can't via the diplomatic channels.

The leak was on purpose.

OK, on the face-of-it: some cloth-heads in sandals, (with blue eyes, and secular society actually, never-mind) but without a navy. They managed to blockade Global Trade through the Red Sea (and Suez), but not for non-combatants - so Chinese ships, no probs. But, Israeli or Amerikan ships would get harrassed, especially the fat, juicy targets that were US NAvY, ostentatiously sailing off the coast. The message was: stop the genocide in Gaza, and we will stop. And to their credit, the 'Houthis' ceased the naval harrassment as soon as Trump declared a ceasefire. The problem was: the merchant ships could not get insurance for the Suez trips, anymore. Second problem was: Israel started firing on Gaza again.

So actually, it was a conspiracy of insurance companies, being all risk-averse, more than anything - say, for example, Lloyds - in London.

So on the face of it, USA 'has to' strafe the tiny drone-operators, as they hide in tunnels underground, and Vance was openly stating it was a bad idea - this may be positioning him as the 'one who understands'. He was saying that the 'strike' might give the wrong message - [to whom?]. Prolly to Russian negotiators in Rhyadh, only just up the peninsula.

Sooo, just out on a limb here: what happens if US attacks IRan, and that country (much more military heavy-weight than Yemen, and backed by Russia AND China) blocks the Hormuz strait, stopping up a second major artery in global oil-trade?. Answer: Nothing Trump ain't that backwards, although there's a bunch of 'em that are still beating that war-drum.

My advice to the warpigs: don't. We are already bruised from all the global shenanigans. We don't need a $300 dollar oil right now. And , I think b is right: The message was:

[imaginary quote] You Europeans care more about the Red Sea Traffic than We Americans. Maybe we should care for our own human resources and security: If you don't play the game, we will stop 'helping'. And you should give up with the war-mongering speaches, (without the military to back it up -you look ridiculous), and also the election meddling (this is Vance we are talking about), before your streets explode with protests due to inflation.

ALso:

Why would high administration officials, who have access to more secure communication systems, use Signal to chat with each other?